Well, IANAL, (and am lacking sleep) but remember, for stuff you wrote, you control the copyright. If it happened before you were employed as a programmer at this company, you own the code. So what that means is that you can say, "This code is GPL, but company X can use it in whatever way they see fit" And this does not break any rights with anyone. You could even say, (if you have manager/boss that's somewhat open to the idea of oss), 'This code is GPL, but company X can use it however they see fit for x years"
Unless I'm completely smoking something strange, MySQL used something along the lines of the first example for quite a while, and still might for that matter. (you have to pay them to become a company X)
As for #2, as n0nsensical put it, "f you made modifications on company time, and then go back and make the same modifications to the GPL version, you could be infringing the company's copyright on what you wrote at work"
This is dangerous. Make sure you have something in writing that says you can do this before you do. If you seriously want to retain control over the project with no potential legal hassles at all, don't mod it at work, let them know that unless they are going to pay you to work on a GPL project, you will continue to add to it on your own time and retain control. It really comes down to you and how much you like OSS.
