Does taking keys off of a MacBook keyboard void warranty

No, rEFIt is software and therefore should not affect the hardware warranty.

However, if you're concerned about that and want to avoid trouble (there's always the odd thickheaded customer service rep, even at Apple), you can simply wipe your drive and do a clean OS X reinstall before you send your machine in for service (and restore from backup once you get it back). rEFIt does not modify your EFI or anything else, so there'll be no trace left of it if you do so.

No - a simple and properly done RAM replacement does not affect warranty coverage. You can have Apple do the work, or an authorized service provider to do the work or hire someone. Just understand what is covered if something happens before you pay for service.

When someone breaks a mac when it's used in a way not intended, that can result in a denial of warranty service. Also - using aftermarket parts that then damage your mac can also land you in hot water. Luckily with RAM, as long as you don't physically break the mounting brackets, usually the worst thing that happens is your mac is unreliable or panics until you get proper RAM.

The user manual for each Apple product will clearly explain which parts are user replaceable and which are not. You can look up your manual online if you care, but no portable machine other than the MacBook Air has ram that is not user-replaceable.