His Informed Ability as a tactician and command leader is just that; informed. However, despite being enslaved in the mines, the Remans do work with the Romulans on a daily basis, even if unwillingly... and above all else, Romulans are known as masters of deceit and lies. Bit-by-bit, the Remans could learn enough to emulate these strategies - so who's to say some other tactician led and won the mentioned Dominion War battles, was killed off, and the records were altered to make Shinzon take his place? With enough support/threats/bribery, they could also back up the illusion until it wasn't questioned anymore.
Not that informed really. He needs Picard alive, so he's obviously not shooting to kill on the Enterprise (only after the collision does he decide to go with a Taking You with Me approach, since he's basically dead anyway.) Notice that he dispatched the two Romulan warbirds in fairly short order, including one playing dead trick on the supposedly seasoned captain.
Except that none of this justifies any of his obviously bad decisions. It doesn't explain him not destroying Enterprise after capturing Picard, giving himself away by mind raping Troi, flying so close to them in battle he loses tactical advantage of his invisibility, not reacting at all to their attempts to ram them (which he should have at least heard about during their war with Dominion). He also shoots the bridge way before the collision happened. He is very much a General Failure.
The rapidly-changing plan, first wanting to liberate his Reman brothers, then proving something to the RSE, and finally going to prove he's better than his clone. Well consider this: he was engineered as a clone, with temporal RNA sequencing designed to make him skip 30 years of his life, matching the original... which was never activated, causing a breakdown in his cells. And like the base of a pyramid, if the base started to become increasingly unstable, wouldn't it disturb everything above? Both physically AND mentally? Without the telepathic stablization from his Viceroy, one wonders if he'd still be sane at all.
In short, Shinzon is exactly what he appears to be: essentially a teenage boy trying to act like a man, trying to prove he's better than the original, AND desperately trying to prove his worth period before he expires. This with all the impulses (and desires) that come with that phase, summed up in the appropriate line "All I have are my personal feelings". And we see him steadily come apart as the film goes on.
Exactly; Shinzon does have a good track record, and he was a skilled military commander in the past, but at this point his cellular degeneration has left his mind just as unstable as his body, and he's desperately hanging on to what sanity he has left so he doesn't become a drooling, screeching mess at the cost of most of his tactical expertise.
B4 may initially jump out as a completely random and/or contradictory part of Data's backstory, but as it turns out, he was indirectly referenced as one of three pre-Lore prototypes built by Dr. Soong.
Why did the photo of Picard as a cadet show him bald when later episodes established he had a full head of hair? In The Best Of Both Worlds, it's mention that Picard ran the Academy Marathon. He could easily have shaved his hair off while training for the event, and the photo was taken during this time.
Wesley went to a wedding where everyone is naked with his own mother. Try not to let that one sink in whilst eating...
Ew.
Although, while it has been portrayed somewhat inconsistently, twenty fourth century humans don't seem to have quite such an overdeveloped nudity taboo as twentieth/twenty first century Americans.
