You speculations are really amusing
I don't see how Tyrion could have done it. If you rethink the whole scene, the camera was focused on Tyrion the whole time and we saw him every time he poured wine for Joffrey.
Tywin? You really think the man can kill his grandson?
Oberyn had nothing against Joffrey. His grudge is with Tywin and The Mountain and they are the reason he came to the capital.
The wedding is done and Margaery is now the queen. Joffrey's death cannot change that. She will remain queen.