Regarding Ultimecia being a bland villain: it's a question of tastes. Someone who prefers its villains to act directly instead of indirectly will find her to be bland, as you don't see her true self until the very end. However, much like Zemus, Ultimecia relies on her ability to essentially project her consciousness to act as she wants, which is why you believe at first that Edea is the main villain. Had they revealed Ultimecia's identity at first, it would have ruined things a bit: the moment where Edea acts as a guest party member and how everyone reacts towards the idea is a key point in the story. It's awkward to cooperate with someone who, until recently, was your enemy; its mind-boggling to cooperate with someone who, until a certain point was basically everyone's mother (save for Rinoa, and Squall of course) and who built the SeeD essentially to defeat Ultimecia in the future by setting certain things into motion. By all means, Edea should qualify as a benevolent chessmaster, facing someone with far more experience and power than herself, and having only a mild advantage of knowing that she'd be possessed at one point and other time shenanigans.

Regarding the Ultimecia-Rinoa hypothesis/"Squall's dream": it's less "Square Enix neither confirms nor denies the idea is wrong", but rather "Squeenix couldn't care, but found it so frickin' hilarious they're throwing a bone at it". Though, it could easily be that Squeenix didn't found it hilarious, but found it so well thought that they decided to give it a mention.

Regarding Jenova-as-puppetmaster: first time I hear about this, mostly. I believe the consensus over here is pretty clear on who's ruling who.

On brute force versus slyness: I find Exdeath to be the epitome of brute force, or at least the term should have been used instead of it. I barely see Sephy as someone who'd rely on brute force, particularly when "brute force" and "surgical precision" are used in the same post (the latter isn't mentioned by name, though, but "scalpel" brings that idea into mind). Brute force and finesse are commonly considered anti-theses of each other. To use brute force is to apply as much force as possible (regardless of area), while finesse implies using the right amount of force in the right spot.

I'd say that "hands-on approach" is a better term, and in here Exdeath excels a bit more. Unlike Zemus and Ultimecia, who do their actions by proxy (or by possession, in case of the latter), unlike Kefka who uses gambits (and even then, he personally takes the time to poison Doma and is present when burning Figaro; isn't that 'hands down approach' at its finest?), and unlike Sephiroth who puts a plan into motion but cares little for his pursuers, Exdeath personally goes and attempts to stop the heroes before his plans are stopped (and when he's defeated, he still manages to tag along and continue with his plan), which is the clearest example of "hands-on approach" I've seen.

I wouldn't say that Sephy went for a brute force or hands-on approach for a variety of reasons, but the biggest one is that he relies on others to do what he wants. He's more of an overseer than a worker: if he'd want the Black Materia, he could have...I dunno, send one of his clones for it. That's more of a "hands-on approach" than anything else. He simply seeks to claim the prize afterwards, which is the extent of his direct actions in this case. On the famous "Aeris dies" scenario, he could have simply done the job himself, but instead of going for the brute force approach ("do the damn thing already! Heck, do it before she gets to the Cave of the Ancients in the first place!!!!!!"), he goes for the mind game instead. Certainly, that's not indirect approach to solving the problem (it's quite direct), but it's not the most efficient. However, he relies on Cloud to attempt the job first, rather than doing it himself. Perhaps this isn't the best example of "in/direct approach", because the nature of the act inclines towards something different (the important bit is for him to toy around rather than solve the matter with efficiency), but it's still useful as the most direct approach to the matter would have eliminated the need for the mind game entirely.

If anything, I insist the worst villain, hands down, has to be Necron, and that's because he literally appears out of nowhere (or at least, as literal as "oblivion ~= nowhere") and has no clear anchor to the story other than "this is what happens when you break the Crystal; nice job breaking it Kuja!" The character development they gave him at those final hours (when he manages to artificially induce himself into Trance, brutally boosting his power) became somewhat irrelevant when he wasn't made the true final boss.

Oh, and this is a gift for Xanatos: note that the party never truly defeated Kuja. It's the opposite: Kuja killed the party, unambiguously, as they wouldn't be capable of facing the embodiment of Death in Gaia without actually dying. Take that as you will.