Mac – Deauthorise Apple ID from the Mac App Store

There are a couple of gotchas I'm aware of.

1. If you buy apps on both iTunes accounts, they all get listed together when updates become available, not just the ones on the account you're currently signed into. So if you click 'download all free updates' iTunes will only download the updates for the account which is signed in. This can be confusing because you've told it to download all free updates and it doesn't tell you why it hasn't downloaded the others.
2. If you have an Apple TV and you want to use Home Sharing, then you can only share the content for one account at a time.

Yes, although there are many questions that cover the same ground already on SE.

You can DL app store apps (including Lion) on your girlfriend's computer. Then she can log into her own apple account and buy other apps. The only thing you need to be aware of is that for updates, you can only update the apps you bought with that particular account. So if you want to update Lion (not likely as it will come through software update) or other apps, you will have to re-log into your account.

I believe most apps have a 5 computer limit.