Attack Federation fleets? Destiny establishes that thalaron weapons work wonders against even large fleets.
We also don't know when Tal'Aura et al. learned about the thalaron weapons. Weren't they Reman technology?
Tal'Aura & co knew about the thalaron weapon from the beginning, when Tal'Aura used it to kill the ENTIRE Romulan Senate - she's all heart, isn't she?
About the thalaron weapon's usefulness - take it up with Picard. He said that the weapon will be used to sterilize planets. My guess - the weapon is ineffective against fleets because, unlike planets, fleets are maneuverable - they can scatter or go to warp long before Shinzon's weapon was charged in X minutes.
In "Destiny" the weapon should be effective against a fleet of borg cubes because the borg don't do "evasive maneuvers". And, perhaps, starfleet's weapon charges a lot faster than Shinzon's.
The evidence seems to suggest that Tal'Aura and Suran thought things were rapidly escalating out of control, from militarist coup to war against the Federation to genocidal campaign against Earth. Proof? Donatra was allowed to leave Romulus with her ships.
Your interpretation of the text is coloured by your intent to make the romulan conspirators look as good as possible.
My interpretation - which, according to Occam's razor, is correct:
Donatra was allowed to leave with her ships because Suran & co were worried about spending time in the dilithium mines.
And Suran sure wished Shinzon to complete his mission: "But can he complete his mission?".
About the gorn.
In "Arena", they killed thousands of men, women and children for no good reason.
They scanned the colony - they knew they didn't need the element of surprise. They monitored federation communications - as evidenced by their use of transmissions to trap the enterprise. At least, they suspected that the federation didn't know about the gorn's existence.
They sent no warning, no ultimatum, noting. They just killed everyone.
This makes them extremly agressive.
There's no question that the Gorn are highly aggressive in the defense of their territory. But being different from the Federation doesn't make them evil monsters bent on the Federation's destruction. If you think that different automatically means evil, then you need to go back to the beginning of Star Trek and start over.
Of course, being different does not make one a monster. Just...different.
But, you know, I have my own values, and according to them, killing thousands of people (including noncombatants) for a flimsy reason does make one monstruous.