1.) There's a hack on
SimWardrobe that'll put an end to spontaneous combustion. Click on 'Sims 2', then 'Fixes'. The mod you want is called 'No Random Bursting Into Flames', near the bottom.
2.) Immunity to fire is just that; immunity to fire. Your Sims won't catch on fire.
3.) I have no idea, never seen a mod/hack like that.