You're welcome. Feel free to hit the "Thanks" button on any of my posts that have been helpful. 
This is the part you may need to "phone a friend" on lol. I was blessed with an early build phone that has never lost the 3 button dealy-o and is almost unbrickable.
GB- changes the three button to this:
Up volume+power = Recovery
Down volume+power = Download mode
For ME- using regular Odin One Click restores my three button deal, but that's because I had it before I started flashing/modding.
Also, it's my understanding that GB bootloaders are backward compatible to older android version (which theoretically would allow you to keep your easy access to DL mode) but no devs (to my knowledge) are building older ROMs that accommodate this ability other than Cezar (see link below).
I recommended you read, at minimum, the section in post one about returning to stock. It's good info and has the link to JF6 2.1 zip that has no bootladers and allows you to keep the GB BLs and working DL mode. http://forum.xda-developers.com/showthread.php?t=1045160
Lastly, there are people like you that have had 3 button issues that run GB and have gone back- I just don't know the story because I've never had to worry about it. Some more research is in order for you on that one...

One last thing (and THANK YOU for all your help!!!)
Let's say I do all of this (go to Serendipity VII and back to stock.) After that, do I need to use 3 button fix to go back to stock from a Froyo based ROM? Or did it fix the three button problem?
This is the part you may need to "phone a friend" on lol. I was blessed with an early build phone that has never lost the 3 button dealy-o and is almost unbrickable.
GB- changes the three button to this:
Up volume+power = Recovery
Down volume+power = Download mode
For ME- using regular Odin One Click restores my three button deal, but that's because I had it before I started flashing/modding.
Also, it's my understanding that GB bootloaders are backward compatible to older android version (which theoretically would allow you to keep your easy access to DL mode) but no devs (to my knowledge) are building older ROMs that accommodate this ability other than Cezar (see link below).
I recommended you read, at minimum, the section in post one about returning to stock. It's good info and has the link to JF6 2.1 zip that has no bootladers and allows you to keep the GB BLs and working DL mode. http://forum.xda-developers.com/showthread.php?t=1045160
Lastly, there are people like you that have had 3 button issues that run GB and have gone back- I just don't know the story because I've never had to worry about it. Some more research is in order for you on that one...