The bit about the Projects folder was a stab in the dark. Don't worry if you don't have one -- if the game/Bodyshop needs it, one will be created.

Using CI to get rid of empty packages, duplicate files, etc. is a Very Good Thing and can noticeably improve game loading time and gameplay. (Standard caveat: using Remove Files in CI will permanently and irretrievably remove files -- there is no Undo nor any other way to reverse the Remove.)

As for your other questions:

Ground terrain covers do belong in Downloads.  sc4 files are custom neighborhood files.

Walls, floors, etc. belong in Downloads. Rule of thumb: if it is a package file, it generally goes in Downloads.

Silly question, but are you sure you're trying to open the Bodyshop affiliated with your highest installed EP (which may not necessarily be the most recent one EP you installed, if you by chance installed EPs in an order different than the EA/Maxis release order)?

Do you have the HCDU (Hack Conflict Detection Utility)? If not, get it here (Sims 2--Programs):

http://simwardrobe.com/

Then install and run it (takes a while to scan your files). Note: Any conflicts it detects may be absolutely benign or may be of a nature to negatively impact the game. There is no way to differentiate between harmless conflicts and nasty conflicts, but the listing of potential conflicts the HCDU spits out can be a good starting point fas to where to look for possible problematic combinations of files.

Other than that, there's a ton of Bodyshop help tips here:

http://www.simswiki.info/wiki.php?title=Game_Help:Body_Shop
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One thing you might give a go is to go to My Documents--EA Games--The Sims 2--Projects and delete the contents of the Projects folder, if there are any (don't delete the folder itself).

Bodyshop projects get put there, and possibly there is a corrupted or otherwise troublesome file in there which is glorking things.

OBJ files (which are a type of text file) usually come from SimPE, and can be moved out of Downloads. PSD files are (if I recall correctly) Photoshop files and can be moved out of Downloads.

BTW, if by any chance you have any jpg or gif or txt or Sims2Pack or html or zip or rar files in the Downloads folder, take them out. They don't belong there (and the game ignores them), but they still bloat the Downloads folder.

Also would suggest running Clean Installer (it can take quite a while to scan the entire Downloads folder - for the benefit of any who haven't used CI, you can tell when the scan is done when the number of files appears up top under the name of the file being scanned), then click the 'Type' button and see if there are any files in Downloads listed under the type labeled 'Collections' -- these should be moved to the Collections folder. This can be done within Clean Installer by right-clicking on each such file, choosing Move, then Browse, and then browsing to the My Documents--EA Games--The Sims 2-Collections folder.

Another possibility (just trying to cover a lot of bases in one reply) is that you might have inadvertantly placed a custom terrain file (sc4) in Downloads instead of in the SC4 Terrains folder.

Used to be a solo buyable obstacle course at SimRepublic, but that site is long kaput. So, another option--

Buyable versions of all the reward objects:

http://www.modthesims.info/download.php?t=374017
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The only way I'm aware of is to use Theo's plug-in (addition) for SimPE:

http://www.moreawesomethanyou.com/smf/index.php/topic,8266.0.html

This removes all the clothes in the wardrobes, so you will have to re-obtain any you do want.

As with anything involving SimPE, it is always best (and cannot be stressed enough in its importance) to do a complete back-up of your My Documents--EA Games--The Sims 2 folder first (and store it someplace safe) as SimPE is a very powerful tool and does contain the potential to inadvertantly do things which might glork your 'hood or the entire game.

SimPE can be obtained here:

http://sims.ambertation.de/
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2 mods which might be helpful (use one or the other, but do not have both in your game simultaneously).

Faster harvest (free reg. req.):

http://www.modthesims.info/download.php?t=229418

Instant harvest (free reg. req.):

http://www.sims2workshop.com/viewtopic.php?t=2468


Some custom pet careers:

http://www.modthesims.info/browse.php?f=229&showType=1&gs=1&threadcategory=Pet%20Career
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What you may be after is not a mode for the careers (which would be a global mod), but just a way to make any adult Sim a college graduate without attending college.

The Counterfeit diploma by christianlov will allow you to do just that:

http://www.modthesims.info/download.php?t=289704

After the Sim has been given the status of graduate, he/she is treated exactly the same by the game as an attending graduate would be.

All of VampCats holdable weapons are here (free reg. req.):

http://www.sims2workshop.com/viewforum.php?f=36

Also some larger (and deadlier) weapons at SimSlice (particle weapons - $$$ items):

http://www.simslice.com/thesims2/guests/wintermuteai1-pg8.htm

Or at the booty:

http://ls.paysites.mustbedestroyed.org/booty/ts2/simslice/wintermuteail-guest_artist/particlegun-simincinerators-2/

Here's how:

Hover your cursor over the link you normally click to open the game.

Right-click on that link.

Left-click on Properties in the pop-up menu.

In the Target Location box, go all the way to the end of the location shown there.

After the final quote mark shown as a part of that location, type in a space, then the minus sign, then a lower case w. The very end of the address in the location string will then read thusly:

.exe" -w

Click OK.

That's all there is to it.

One of the things that pops up in the game is that, from time to time, the wrong thumbnail images show up in CAS and/or the catalog. That sounds like what is happening in your case (it is nothing you did - it just happens).
You can refresh (and correct) such incorrect thumbnails individually by using Control-right click on the thumbnail image. (Doing so on a correctly shown thumbnail will have no apparent affect - the refresh will continue to show the correct image still.)

You may discover that your 'missing' Sims are there in CAS, but that just the thumbnail image showing in CAS was incorrect.

I don't recall if that method also works in Bodyshop. But since Bodyshop accesses the same thumbnails as does the game, if you fix them in CAS they should appear correctly thereafter in Bodyshop.

Personally, I've even occasionally had thumbnail images of males show up in the female section in CAS (and vice-versa). Using the refresh thumbnail image trick always corrects the wrong image.

Using one of the powerful mods/mod devices out there (for example,  Insimenator, or any such type of mod which controls/affects flags), you can set or remove relationship flags (engaged, married, love, furious, etc., etc.) at will with as many other Sims as you prefer.

Note: unless Lot Jealousy is also disabled, then relationship jealousy and its concomitant jealous behavior will likely ensue.

Also note the now required statement that Insimenator is no longer supported.

If reflections is enabled in the game, you will get as much totality of reflectivity as your graphics card/graphics chip is capable of providing in the game.

You cannot increase or further change the amount/area of reflectivity unless you replace your graphics hardware with something more game-robust and able to handle fuller reflectivity.

That said, updating your graphics drivers may allow for slightly better performance with existing graphics hardware.

Reflections being enabled does add stress to the graphics card/graphics chip, which in some cases can lead to game lag, freezing or crashing, and in the case of more limited graphics hardware can lead over time to overheating and incremental and permanent damage (or even failure) of the graphics card or chip.

Not from komosims, but certainly reminiscent in many ways of some of the palaces of St. Petersburg:

http://www.modthesims.info/download.php?t=211592
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Maybe these? (Free reg. req.)

http://www.sapphiresims2.com/showthread.php?t=26891
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What you're after already comes with the game. It just takes some one-time organizing to make your own in-game collections.

Organize your stuff as you prefer into collections folders inside the catalog. Then you won't have to go through all the walls in the catalog but just go directly to the collection folder you made with only the graffiti walls in it (for example), and so on.

If memory serves correctly, then yes, the vampire bat edibles will work when placed on community lots and are accessible through the catalog in community lots (found under party items, if I recall correctly).

However, if it is daylight then the usual vampire caveats apply, and anyone who eats the 'become vampire' treat  faces the possibility of ending up as a heap of ashes if unable to flee the lot (or eat the 'de-vampire' treat) soon enough.