on the PC, it's pretty safe to store items in houses you take over... on the xbox, houses/chests have a limited number of items they can hold, after which an "overload loot bag" (i think) spawns and keeps your items inside... sometimes the items in the loot bag vanish
A) On PC, you can download mods that have houses, there is an awesome bank mod that allows you to buy several houses through the bank. Indybank Mod is the bank mod, I always use this one when i play. Has alot of really nice features.
B) Yeah, if you kill someone and take over their house, you can throw crap in there to no end, and I dont recall the chests emptying at any point.