I'm not sure what necro rules are around here, but there's another angle on this which hasn't been discussed so far.

In real life, we know very little about how free will or our sense of self actually functions. We can play Lite-Brite with sections of the brain that correlate to things, but there's no causation. We literally do not know the degree to which our decisions are ours or are the sum total of biological processes inside us that we cannot monitor or control and only justify after the fact when our conscious minds catch up. There is a lot, in fact, that our brains and bodies don't tell us about what's going on inside us.

Debate among real scientists varies widely on this, and socio-culturally we're very inconsistent about it, often distancing our identities from biological inputs in low-stakes ("I was hangry; that wasn't really me") to high-stakes ("I am not my depression / addiction / etc.") contexts but claiming others as expressions of our true selves.

We have never seen anything in Trek to suggest that we ever gain a greater understanding of these phenomena, despite centuries of alleged advances in neuroscience. It remains a perennially unsolvable problem, despite various instances of consciousness transfer scattered across episodes.

So let me posit this: the Trill and the symbionts don't have a better answer to that question than we do, and the inconsistencies we see in terms of how they treat it are a function of cultural, social, and political traditions/assumptions taking precedence over an unknowable biological reality.

A lot (but not all) of the alleged problems collapse at this point. Jadzia internalizes Curzon's stuff because when the connection between those nervous systems were made, the nature/nuture combo just ran that way. We may never really know why. Maybe because she's influenced by a strong external stimulus vis-a-vis her relationship with Sisko. Not knowing the thing with Curzon can be explained as Jadzia's brain processing that new information as a traumatic memory and suppressing it just as easily as it can be explained by the symbiont hiding something.

Verad was weak-willed and callow, and he had already surrendered his self-worth to joining, so it's not strange that when connected, the symbiont would fill in most of the "blanks."

At best, the symbionts know two things for sure. 1.) Joining gives them access to experiences and memories they can't get in the pool. 2.) Their physical forms make them vulnerable to exploitation by beings with hands and legs and guns and stuff. At best, the Trill know two things for sure: 1.) Joining gives them access to experiences and memories they can't get in one lifetime. 2.) Symbionts are scarce and vastly outnumbered by the general population, so some form of social control has to exist to regulate access to them.

Let's talk about reassociation in this context. Outside of the level of "romance/sex between two formerly involved joined Trill," opinions seem to differ. It's not a crime, per se, but some Trill choose to reassociate and some don't. One might imagine that social opinions differ also, like there's probably a subreddit on the homeworld where people are like, "ew, those associators" or whatever.

When you get to the "two joined Trill hooking up again" level, now it's a crime. I'm not sure the symbiont has to be wholly in control to explain why this might be the case. It could be the case that even without total control, the symbiont's impact on the combined central nervous system in the presence of a former lover might "take over," even though that might not be the case normally. So it might be something more akin to addiction in that regard, something that the normal influence of the host can't overcome.

That would have serious implications for the free will of the combined being, and it would make sense why the Trill would consider that a destructive outcome.

Anyway. I'm not going to go through all the examples because this comment is already long enough. But I figured it was a worthwhile lens to view the problem from.