It is absurd to say Barca was the reason for you to not help Terran and Asylon. It was proved that Barca could complain all it would like because of the regions you stole, but nor in diplomatic neither military way it could do anything to get them back. Did you see Barca as a threat? At this point it sounds just ridiculous. And we still have got the Provincia di Fiorenza matter. That was an awful way to prove your good intentions towards Terran and D'Hara.
Barca threatened future war and hostile actions against Aurvandil, whilst insulting the High Sovereign and telling Aurvandil we owed our existence to them and we should do as they say on that basis, yeah that didn't go down well. You can call it absurd, but Aurvandil likes to be SMA about diplomacy, and insulting and threatening a monarch is not something that would have been tolerated by a foreign realm.
In "single combat" Fissoa could have beaten Falkirk. And I think you underestimate your enemy if you think a combined force of Fissoa, D'Hara and Luria Nova couldn't put you in a tight spot. Moreover, I don't think you could indefinitely support economically and in food supply Falkirk. You have already squeezed your regions and pushed the peasants to their limits, and maybe a new war declaration from Luria Nova and D'Hara could induce to a civil rebellion.
If they could have beaten Falkirk, they would have, but as you have seen Falkirk consistently destroyed and defeated the Fissoan army in the field more than five times. The Fissoan armies were being consecutively destroyed by Falkirk every time they marched over. Oh I'm sure they could put us in a tight spot, but we know our !@#$ when it comes to war. Falkirk especially, as they are all veterans of the League of Free Nations and the continent wide war, they've shown they can take down bigger, stronger enemies despite being in a tight spot.
And civil rebellion? I doubt it, not now that Aurvandil has a lot of diplomats, courtiers, priests and indigenous religion, plus enough nobles that allow us to police and work on our regions whilst still being able to send over 50 nobles on campaign.
Maybe in a short term if Falkirk falls it would be beneficial for you. But your lands can't produce enough gold and food to maintain the army that would correspond to 90 nobles. At some point, they wouldn't just be able to pay their troops, and of course it would help to block the eventual commerce you could have with the current Falkirkian lands. They would have to start using their own family wealth and if there is actually some kind of rivalry between you, maybe they would start to be willing to return to the lands they owned although they were called in a different way and sustained a new realm.
You underestimate what Aurvandil can do on a limited budget, and food? We're rolling in a surplus of thousands even after supplying the Freestate, we won't be short on food at any point due to maintaining an army, and we don't recruit to the maximum at all times, we reserve the bulk of recruitment before we go on campaign so our forces aren't sitting in the capital eating gold and food needlessly.
And the Falkirkian's are professionals, that means we can put aside our personal rivalries when our duty/honour/mutual causes call for it. Plus, Aurvandil would re-found them a realm anyway, Mendicant doesn't like the Averothoi, he respects them, but he doesn't like them.