The problem is that Ardan has no claim on the Throne. Victor DID have the claim and was simply deposed by Katherine *spits* and in turn had Peter jump in after her. Victor did renounce his claim, but if he was asked to come back, in all honesty he would have no real question marks about that as he's eldest child of Melissa after all.
Ardan is a nice guy, but he's not a Steiner.
Victor OTOH has, since the Federated Commonwealth broke up, learned some basic political skills on one hand, been proven 100% right about his sister on the other hand and been given the ultimate vindication of Kings for thousands of years; he's f*#king owning everything and anything that comes up against him or his nation, which has thrived like never before.
The Lyran Alliance OTOH has been steadily going backwards. Poor Peter even kept getting called a 'second rate Victor' in the media, IIRC, whenever he managed to do genuinely good things, simply because Victor had already done it and far far better.
He might have taken a while to get there, but to the average person in the Federated Suns and Lyran Alliance, Victor is the ultimate blend and evolution of Katrina+Melissa+Hanse and perhaps the greatest leader a Successor State has ever had.
The fact that its entirely possible a majority of the Estates Generals would have outright voted for Victor to come back and rule them in a straight up/down line despite everything shows that A) They recognize his greatness B) They understand (with the exception of a handful of representatives who are probably not going to make it off Tharkad alive) that he was innocent of all the charges against him and that it was the traitor Katherine all along who betrayed Melissa and destroyed the FedCom and C) That they don't have any tenable leader right now. At all. And their situation is looking increasingly desperate.
Of course, the population of the Federated Suns have a lot of reasons to be highly skeptical. Where as the original Federated Commonwealth arguably was a joining of different strengths (FedSuns competence + Lyran industry) now the Federated Suns have BOTH. They also have the sheer vindication that it was the Steiner side who blindly followed a matricidal bitch and turned their back on the Suns. Who stole the Federated Suns jumpships (and then SOLD THEM BACK TO THEM). Who ranted about how the Federated Suns wasn't pulling its weight against the Clans (despite the fact that AFFS units in the AFFC were actually a plurality or even a majority of the units who fought the Clan invasion!) and whose armed forces, the LCAF/LAAF all turned their back on Victor despite the fact that HE fought his ass off in the Donegal Guards and Lyran Guards against the Clans defending that half of the FedCom, arguably at the cost of the Suns side...
And Victor himself, IIRC earlier in this fic, even admitted to a sense of personal betrayal at how rapidly the Lyran side of the Commonwealth turned on him despite how hard he had worked and fought to protect it.
The idea that the Federated Suns would loose quite arguably the finest First Prince they ever had to Tharkard would probably make a LOT of people nervous.
Victors best bet is probably going to be to take a third path here. Formally reject the idea of him becoming the Archon Prince again. But, support someone competent as a regent for his Nephew everyone can respect - while making it clear that he will be working to increasingly strengthen ties, formally, between their nations. I'd suggest Victor nominate Roman Steiner. Unlike in the OTL, he didn't loose his granddaughter to the 1st Davions last stand on New Avalon fighting for Victor nor almost loosing his other grandkids on Tharkad to Peter, so I'm going to guess he's still a firm supporter of Victor and at least thought well of Peter.
He has the family position and exceptional connections in both states to be an effective regent.
As for the FedCom, again I'd say take a third path. Review the original FedCom treaty and look to revive the useful parts. No union, but see what can be done to once again open up economic, technological and military ties like they did in the 3020s-3030s, re-establish a free trade pact and open up markets, technology transfers, all of that stuff. Formalize a level of alliance quite a bit short of a union, but closer than they have right now, getting the maximum win-win advantages for both sides.
And of course, far closer military cooperation with an exchange of high level staff on both sides to start fighting this new war together.
On a different topic, it looks like the Nova Cats are about ready to jump ship. This is going to get very interesting, very quickly...