Where can I read about this Timo's alternate explanation? It sounds really interesting!
Oh, it's more like a hobby horse than anything else. But here goes...
Why send an invasion force against a foe too mighty to be defeated? Classically, there are three reasons: to keep the enemy off balance (like with the raids at Dieppe or the fisheries of Norway in WWII), to gain the respect of potential allies (ditto), or then to get rid of unwanted elements in a formally acceptable way by forcing them to participate in a suicide attack (a theme repeated in many a medieval battle, particularly in Europe and Japan). The Romulans might have combined all three here, in a true win-win scenario.
Say, they know they have this nasty underground unification movement going. They fear losing the respect of their allies within the Star Empire and abroad. What do they do? They foster the movement, encourage it to act. They provide it with whatever it needs to conduct a peace mission to Vulcan, including vessels stolen from the Federation. They load the peaceniks in the stolen vessels and send them towards Vulcan under peace signs - with cloaked Warbirds full of troops riding in the turbulent wake of the noisy old tubs, thus perfectly hidden from the Federation border sensors.
Are the peaceniks allowed all the way to Vulcan? Good - then the invasion can commence, striking fear in the Federation and earning the admiration of Federation's enemies. Vulcan can be held for a while at the very least, with hostages taken, "political rearrangement" performed with disruptor and blade, and a permanent mark left in history. Or then the tens if not hundreds of thousands of troops in those Warbirds are a force large enough to take and hold Vulcan for good. All in the name of unification, with the peaceniks getting the brunt of the blame (although conveniently perishing in the first waves of attack).
Are the peaceniks intercepted? That's not too bad, either. The Romulans then go with the pretense that they were the invasion force, scuttle the transports, and the Federation actually cheers when two thousand dissidents are blasted to oblivion! Potential Romulan allies are still impressed with the audacity of the failed invasion attempt, and the Federation is shaken with the impact of how narrowly the disaster was avoided.
Win-win for the Star Empire, lose-lose for the Federation and its attempts at peacefully manipulating the Romulans.
The best part is, this is exactly what we were let to understand in the episode - that two thousand peace envoys would seek contact with Vulcan. The only reason we ever thought they might be armed invaders instead is because
the Romulan villain said so to Picard, Spock and Data after capturing them... And clichés aside, who in his right mind would trust anything the villain says, especially when soon thereafter, an opening for escape miraculously opens..?
Timo Saloniemi