Ok I'll read that upI think it's mentioned briefly in the 5th edition Monster Manual and possibly DMG. I know Zariel is mentioned in the paragraph on fallen angels in the Angel entry of the MM. There's not a lot of information out there on the most recent switch.
Mephistoteles looks like a good option...I seem to recall Mammon being antagonistic to Tiamat (competition over the souls of the greedy) if you wanted to go foe. I think Dispater was her ally in Scales of War (in 4e, so I am not sure if that carried over).
I doubt Mephistopheles is her ally, but if her leaving was a blow to Asmodeus, he might be willing to provide a little indirect support with some plausible deniability via a warlock (I just traded power for a soul, how did I know he/she would help Tiamat?)
Where can I find this updated history?? I found some from past versions at some wikis...
I don't know much about tiamat's relationships... Any suggestions??