I didn't think there was an execution carried out for re-association, just the Trill was exiled from the Trill homeworld and so when the Trill died of old age or whatever cause, the symbiont wouldn't have another unjoined Trill available for transplating in time. So Verad would have to find a elderly or terminal joined Trill and hang around with them and a doctor capable of doing the transplant possibly for years waiting for the Trill to die. Lot of waiting, and Verad didn't seem to be very good at waiting. Besides, re-associated Trills seemed to be extremely rare because the sanctions were so strong. And not crazy about advertising themselves, either. It seems to me it would be like advertising oneself as having an incestuous relationship. Lots of people have childhood dreams that don't come true. Kids want to be a doctor when they grow up, works hard in high school and undergrad, but still don't quite make the cut for medical school. Girl wants to be a ballerina and practices for 4 hours everyday from age 5 to 17, but then grows up to be six feet tall. End of dream. Most people don't become murders because their dream has to change.
Plus, Verad picked Dax specifically because of the proximity and access to the wormhole. Much easier to get lost in an unknown quadrant than be hunted down by Trill authorities and Starfleet for killing their officer. Never mind Sisko will go after you. Hawk will hunt you down...
Better to live out your days as a mediocre man than live them out as a murderer, a fugitive, or a penal colony resident. My previous point is valid, though. Whether Verad could have done it or not, it seems unlikely that (in the event of a reassociation) the thousands of other Verads out there would be content with letting two symbionts die just because of a dumb taboo. If symbionts were as rare and sought after as the show implied they were, there would be no way to stop them from finding new hosts aside from killing them on the spot.
I don't know anything about the canon material for how Trills first became a joined species. But it's an interesting question, coming from the perspective of "If I were writing that episode what would I do?" and "What is plausible given our knowledge of evolution?" It could have easily started out like the gu'ald, as an involuntary infection, but became voluntary as time went on. Or the other way around, Trill symbionts could have just been intelligent slugs and early Trill adapted their biology to blend either for some kind of healing properties or some other reason. Or, the symbionts might have blended with the hosts before either was actually sapient as some kind of mutual self preservation adaptation and by the time they evolved sapience it was just normal.
I always wondered if the Trill humanoids are really colonists of a civilization that died out and the symbiotes are the only indigenous intelligent life there. There's dozens of ways this whole idea could be played out, especially since they are one of the only joined species in the franchise.
Something else to think about... A young host may feel they are losing their selves to an older symbiont. In real life, some folks with strong willed friends may suffer the same. My suggestion would for each to be the other’s first, so that they learn together. Now, if the trill should die by mishap, the next symbiont should be much older, end of life. By the time the host is middle aged, a union with a middle aged trill would sync up nicely.
It doesn't actually work that way. Normally, a host receives a symbiont until they die, then the symbiont moves on. Over a 550y lifespan, a single symbiont will probably go through (on average) 8-10 hosts. Dax will have more because her time with Joran, Jadzia, and Torias was relatively brief.
Doesn't the symbiont have its own memories and personality independent of past hosts? Verad suggests as much in "Invasive Procedures" when he states that he believes himself to be a good match for the Dax symbiont because their interests align ("science, diplomacy..."). In that case being a symbiont's first host would still result in a new personality the fusion of the host's mind with the symbiont's, rather than the host just carrying around a "blank hard drive" recording their memories for future hosts and getting no benefit themselves.
I feel like it would be terrifying to get a 'baby' symbiont because you wouldn't know how its personality and traits would affect yours? Like with one that has had even a single lifetime of experience, you would be able to find out from previous host(s) what the symbiont was like and how to prepare for it. But if you're the first host, you don't know anything about how it'll change you and you're stuck with your new self for the rest of your life.
Maybe there's a strict selection process for symbionts too. We don't know how old symbionts are when they're first joined or what requirements there are from their side. It's believable that part of the Symbiosis Commission's job is matching symbiont personalities and interests against potential hosts. Maybe the majority of the symbionts don't get joined either and remain in their pools doing... whatever it is symbionts do there. Edited to fix grammar issues.
In the novels, it’s explained Odan is so overpoweringly dominant because he’s so old. He’s had at least 8 hosts when Vulcans and Humans make first contact.
If, as @Markonian suggests, symbionts become more dominant as they age (Odan being an example), very young ones would not likely affect the hosts' personalities that much. About that, though...the Tal symbiont in DSC is presumably not young (it's currently on its seventh host) and yet it doesn't seem terribly dominant. And later Dax hosts, like Jadzia and Ezri, were clearly not dominated either. My take is that when the Trill were first introduced (in "The Host"), and Odan was the only symbiont we knew anything about, the writers really didn't know what they were ultimately going to do with the Trill. The early concept may have been that unjoined Trill hosts were basically empty shells - Kareel, prior to joining with Odan, kind of acted that way - but this obviously isn't what they truly are (not only have we seen many unjoined Trill living normal lives, we now know that the majority of hosts are unjoined). So it's really just a retcon.
It would be an honor and incredible experience for both the host and the slug; the joined journey would a sight for both and I would think would fun. The tales they'll share and what they'll bring for a future host is pioneering in my opinion. Disappointed? Not at all. It is the birth of 2 lifeforms and glass is half full for both characters.
You're spot on, it turns out! Gray and Adira learned to play cello from Kasha, the first host ("The Sanctuary"). So Kasha's musical proclivity became a trait for Tal's subsequent hosts.
This is a really interesting question. I love there are still ideas/concepts to discuss even though DS9 is 20+ years old. Honestly? I always imagined that most Symbionts at this point have had a host at one point. I guess I didn't think about the fact that new symbionts have to be born. I think you all have great answers, though. While the host may not gain tons of knowledge, they get to be the first and live on with the symbiont. That alone must be a great honor.