It could but it shouldnt be that much of a difference. If it is then you might want to upgrade whatever you use to pick up that wireless signal.

Your connection will be weaker if things are in the way, yes.

Anything that gets in the way of you and your router will slow your connection down. How much it will slow down depends on what it is exactly, and how good your router is. If you have a 20 bucks 54G/150N router and a few concrete walls/ceilings, then don't expect much of your wireless connection. However, a good 300N router can go through walls/ceilings just fine. It will slow down, but you won't notice it as much as with a cheap router.

tl;dr - the better the router and the less walls/ceilings, the better your connection will be.

yes it will but if you have led paint on your walls it will slow it down massively