About metaweapons: Remote controlled ships are just ships equipped with standard weaponry aka they're useless. Kamikaze attack runs - at impulse or warp - have limited usefulness. We know this because Starfleet officers/ships had no problem comitting suicide during the borg invasion. There are at least 3 established cases of a Starfleet ship engaging in a suicide run: in "Destiny", above Kithomer and above Deneva; in 'Full circle', Voyager. Such attacks only worked when there were special circumstances involved - for example, the starfleet ship was out of phase or the borg cube was heavily damaged. Suicide runs at warp are equally ineffective as impulse ones - if they had worked, half of Starfleet would have used them against the borg. If the Verithrax managed to destroy a cube with such a kamikaze run, then either that cube was already damaged by other means or the romulans carried an exotic weapon on board. Thalaron weapons - "Nemesis' established that the romulan military didn't have them. Also, if the romulans had used thalaron against the borg, some federation official or Picard/Geordi would have comented on it during their hypocritical/treasonous posturing. Sunseed - not part of relaunch continuity. Until a book establishes it as part of this continuity, it's just fanon. Subspace weaponry - a plausible candidate. 'Insurrection' shows the potency of such weapons - and their danger: unpredictability. 'Serpents among the ruins' establishes that the romulans have such weapons and used them at least once before. About repercussions resulting from the romulans using metaweapons: All other powers already knew the romulans had metaweapons (subspace weapons, for example). All other powers had metaweapons of their own (for the Federation - Genesis, Thalaon, TOS, TNG establish half a dozen more). This means, of course, that no one should be alarmed about the romulans using metaweapons - and planing to invade Romulus or something alarmist like this. There could be accusations of breaking the various treaties forbidding the use of such weapons - of course, considering the situation in 'Destiny', I doubt they'll be too loud. About a cultural division existing in the romulan empire: Prior to the division of the romulan empire into 2 states, there was never any intention on the part of the scenarists/writers of depicting the romulans as being culturally divided on a large scale. After this division - I guess such a cultural division can be retconed in.
No it did not the ship that used the subspace weapon was who the Romulans were fighting, the Romulans also baned use of these weapons after the planet they were defending and it's possibly billions of inhabitants were destroyed by the weapon. and since the treaty banning them is the same one that keeps cloaks out of Federation hands, the feds get cloaks and subspace weapons so win win for them.
The ending of 'Serpents among the ruins' shows a subspace weapon in action - it was a singularity with assorted subspace technobabble; all of romulan design. Just because no one would blame the romulans too loudly for using subspace weaponry against a genocidal borg fleet does NOT mean the treaties forbidding these weapons are abolished. Also - the federation already has the tech for cloaks and subspace weapons; treaties it signed just forbid it from using them; in the case of subspace weaapons, it most likely has no interest of using them anyway - too unpredictable, too 'immoral' etc.