It's not like Riombara knew about their intentions and decided to go out of their way to give them a home knowing what they were up to. That is just silly. Here is the explanation:
Riombara never affiliated with Invaders. The Meridian Republic did, and, prior to its secession, many of the nobles of that realm advocated for collaboration and tried to get Rio to collaborate. Riombara did not. That's why MR seceded. Because Rio refused to collaborate with the monsters.
And about the Nobles that seceded:
Most of the people who were behind the secession did not return to Riombara. Telrunya and D'Este were given amnesty because of their voluntary actions in returning their duchies, plus Telrunya stepped down. Celyn Haethorne was already gone. Hylor Hobbs was only permitted to return because he was deleting his character and wanted to do an RP about it. Vellos had already sacrificed himself at the Temple of Light. Lefanis was not permitted to return under any circumstances. Athins was gone. The only one I can think of whom we allowed to return was Nigel de la Fere, and Evander pushed to ban him. Thought about trying to get Yuri Ishimu banned too for taking Avengmil over to MR shortly after the original secession.
Guillaume was never "risen up against".
What happened was that Marec was Ruler of IVF and he turned against Guillaume when he learned of his Agreement with the Daimons. He then joined Riombara when the Daimons besieged IVF to continue the fight, where Guillaume then angrily left for Enweil. That IVF under Marec is what Riombara is recreating, because IVF distanced themselves from Guillaume in the end when they learned the truth, which was enough for Riombara to accept.