OK guys, I need help and loads of it! Snow days aren't good when you don't understand something.

Covalent Bonding (or just bonding, I've been told ionic is still classed as bonding for teaching senior school but it isn't really, not in my eyes).

How do the elecrons pair up?

Are they limited to just the space in which the orbitals overlap so both atoms feel complete?
Do they have to be of the same spin to pair up (I think so but aren't sure)?
Do orbitals have to bend to form pi bonds or do the electrons leave the p shell?
And does the sigma p orbital bonding mean both little arms of the p shell have to bend over to match up? Does that make sense? Not really.

And here's an unrelated one. I comprehend how spin keeps electrons in the same shell from hitting each other, but how do s electrons not hit the electrons in the p orbitals? Because the s shell overlaps all the p shells and d shells and whatnot so how do the electrons not bump into each other. OK they repel each other but does that alter the "shape" of the shell, are electrons less likely to be found at certain points, especially in the s orbital, if there is a chance they could collide? Or is this a really, really stupid question?

And if anyone wants to pass this onto a chemist please feel free. I just don't get this and the lecturer doesn't understand my questions! (Then again, I'm not convinced I do some of the time either.)