Heck I agree with you about the Typhon Pact. Oh no it's the
Kinshaya and the
Tzenkethi
. I guess the idea is to us the Pact as an attempt to balance the UFP in the AQ/BQ to which the Feds formed the KAA with the Klingons, Ferengi (stop laughing at the back

) and possibly the Cardassians(still not sure yet

) oh yeah and the Talarian Republic! I must confess to being quite the astro-political junkie so I'm kinda looking forward to this.
Oh no, not the Talarians (these are the child-beaters, right, not the guys who can't speak properly?)!
The Ferengi could be suitably powerful if handled correctly. They came through the Dominion War and (sigh) Borg invasion intact (afaik)... they're probably a major player by this point.
I'm also of the neutral CU that would pose an alternative to the Feds and still cooperate. Why can't the Cardies be the good guys for once

.
Because the Cards were laid prostrate at the feet of the Federation and made to confess their sins.

I still say I'd vastly preferred to have seen them become Fed members.
As for the nature of the Romulan Star Empire(s) I imagine a certain level of democracy for the Romulan citizens( based on the senate) but with a strong intelligence arm and Military industrial complex. The problem is that it is not elaborated upon in any depth so I can't really judge for sure.
Concur.
If whacking a few senators FUBB the RSE that badly then losing the homeworld would have destroyed the empire politically leaving only it's mighty fleets of Warbirds to seek vengeance

.
The subtext that's aggravatingly never made text in Nemesis is that the thalaron-armed Scimitar was hovering cloaked above Romulus for days for a reason, and that reason was to cow the Romulan Empire into submission. Notice that the moment the Scimitar left orbit, the Romulans rebelled. It may be possible that Donatra was feigning it the whole time--heck, maybe the novels tell us she was.
No one else had ever crossed that Rubicon of holding Romulus hostage to ensure the loyalty of his commanders. It took a deranged alien slave to do it, but I suspect it would brutalize Romulan politics in a way heretofore unseen.
And I'm not sure if I've mentioned this here, but the destruction of the "whole Senate" simply could not have happened. In Inter Arma Enim Silent Leges, we see a roomful of people who are on the Continuing Committee--an elite group, the inner circle of the Romulan Senate who dictate their will to the rest of a presumably much larger body. Tal'Aura kills a roomful of people at best two or three guys more numerous than the CC. So unless the Venn diagrams of the Senate and Continuing Committee are identical or nearly so, she didn't kill anyone but the very apex of the executive branch of the government. It annoys me greatly that the Romulan government, a government some two thousand years old, is so brittle that killing a roomful of jerks caused their society to collapse! It also annoys me that Tal'Aura, the woman who delivered billions of Romulans to the tender mercy of Reman slaves, somehow mustered enough support to get a single warbird, let alone hundreds or thousands.
My personal preference would have been for a Romulan/Cardassian alliance, that would have really pissed the Klingons off
Could be neat, and has historical precedent. Unfortunately, we're left with a wasteland kingdom with a functional democratic government and little interesting it's capable of doing and a strife-ridden empire riven by a battle between two quasi-extras, at least one of whom is a traitor to the Romulan people. Oh well.
Apropos of nothing, has TC Chairman Koval been up to anything lately? Or did he die when Tal'Aura turned "the whole Romulan Senate" into mulch?