I have completed the Main Thieves Guild quest - wanted the Nighingale Armor (now it sits on a mannequin).... However... Now Byrnolf won't speak to me - say another time - he's busy.... and in my Misc. quests, there is the item to talk to Byrnoff about joining the Thieves Guild?!!

Will this effect any radiant thieves quests and will I miss out on anything? I'm certainly not going back! I met him waaayyyy long ago!

I don't even know why you still have that Misc. quest. Most likely a bug, but doesn't affect anything.
And Brynjolf not talking to you isn't a bug, just an annoying feature after the questline completion.

Have you been named Guild Leader? Since to become GL you need to do enough radiant quests as well as the Main questline. I think he gets stuck with that dialogue because of Karliah getting stuck somewhere and the Nightingale Hall not working properly (Brynjolf is a Nightingale as well by the end of the questline).

You can still finish all the quests for the Thieves Guild. I think Brynjolf does the same for everyone. There is something messed up with the Nightingales part of the quest line and with Karliah.