They can–though given that they’re using only their own blood, they couldn’t go toe-to-toe with a human Theurge equipped with phials in a major Change-duel. Their uses tend to involve short bursts of super-speed, hardening talons to break armor, hardening natural body armor to repel attacks, etc.
Neither the Federation nor Seracca mating practices are well suited to the concept of a “marriage alliance.” Monogamous lifelong pairing isn’t seen as the paradigm mating practice; and the bonds of federation between clans aren’t dependent on or strongly affected by intermating. So no, hooking up with either Abhuman is not going to establish a political relationship with the Federation.
Xaos-landers call it the Abhuman Gulf. Abhumans call it the Brauracha Gulf.
Absolutely not. 
Deposing the leadership of the empire you actually live in is hard enough. And Erjan has no family ties to the Emperor.
“Helot-led change disguised as aristo-led change” is definitely going to be an option, though.
The day job I’m in the process of handing over is with an ecumenical diakonal mission, indirectly linked by our governance to both the WCC and the World Evangelical Alliance. We also do lots of interfaith engagement and peacebuilding work with Hindu, Buddhist, Kirant, and Muslim religious leaders in our working areas.
There is a big difference between oikumene among different branches of a single faith and interfaith engagement between religious groups that don’t recognize each other as sharing a common set of core beliefs/religious values and practices.
The reach of Christian ecumenism is limited even among Christians. The WCC is primarily a coalition of Euro-American mainline Protestant denominations; it has limited participation from evangelical-Pentecostal Protestantism (which looks to the WEA, to the extent that it affiliates with a global trans-denominational network at all), an often-uneasy relationship with its Eastern Orthodox members, and amiable but strictly limited cooperation with Catholicism.
The different major Christian groupings struggle to turn their shared values into a concrete program of action that isn’t disrupted by bicketing over differences. (The same could be said of e.g. Theravada and Vajrayana Buddhism, Sunni and Shi’i Islam, etc.) It’s even harder to bring unity in an interfaith context – which the Xhthonos/Theoi relationship certainly is. We all value compassion and justice, but as soon as you get past the shallows and look at what that actually means – both how we define those terms, and what the practical consequences are – it’s a lot harder to turn surface agreement into united action.
Peace and tolerance can (must!) be fruitfully pursued in both arenas. But true ecumenical unity is a distant goal even within major faith streams, let alone between different faiths. Don’t expect most Abhumans or Xthonists to agree that “compassion and community” makes them fundametally one.
All population numbers are still subject to change, and thus not being given out.
You’ll have weaker connections to the places you haven’t visited yourself. So if you’ve focused on forging Hegemony-wide ties, you’ll be more vulnerable to a Game 5 crackup of Shayard; if you’ve focused on Shayard, you’ll be less likely to sustain strong alliances across the Hegemony. Your various deputies and ambassadors can mitigate this dilemma but not completely remove it.
Did you ever meet Ganelon again in Ch 3? Not a very strong basis for an alliance, alas. (@idonotlikeusernames, surprised you didn’t react to this one.)