It would depend on resources available to make it viable. It's not just about knowing that a thing exists-it is having the technological resources to continue forward with the research. This is why I added my caveat about Stamets essentially erasing the spore tech and himself from existence.
In a big galaxy with a plethora of races (according to Memory Alpha, Trek has had 542 named races - the galaxy in the 23rd/24th century probably has several thousand at minimum - more likely tens of thousands to hundreds of thousands, given how many species evolved very close to Earth) it becomes almost a certainty that any possible technological breakthrough is happening somewhere right now.
No, but they would have had to discover it first. The spore drive is something we've seen as not being used by anyone else in Trek, now or in the future, and at some point there would not be enough people alive to have knowledge (or enough knowledge) about it to make the Borg or anyone else be able to use the technology.
Again, that's not how technology works. The ancient Greeks invented the steam engine, but it was just used as a toy. Romans developed, and then lost, a type of glass-making which took 1500 years to rediscover. Someone invented a working battery in Sassanid Persia which seems to have been used for electroplating of metals, but it never took off. Technologies which have applications may not be picked up as soon as they are invented, but if they are useful, someone will rediscover them. And if you're dealing with a big galaxy full of competing races, you have many more possibilities for rediscovery.
It could happen. Although if it's done via relatively low-cost germline therapy, there's no reason why a group of elite genetic "supermen" would develop, since it would just mean the next generation of everyone (minus those who had religious objections) would be much smarter and healthier than those who came before). Trek always effed up the augments, FWIW, because they never seemed particularly bright to me (except in DS9).
To Boldly Go Where No Man Has Gone Before... *see disclaimer* *disclaimer* well... technically we might have been there before but records of having been there before were destroyed somehow or it's classified that we've been there before
While I do agree that's true, a simple answer as to why we haven't seen it elsewhere is that it isn't a sustainable technology, caused too many accidents, etc. I mean, it's very likely that any other race that invented something like that would have had the same problems as Discovery did before learning how to effectively use it. That could easily have prevented others from using it effectively too.
As long as the spore drive is functionally speaking an unworkable technology (not difficult, but literally unworkable) than it is fine if it's never revisited. They just shouldn't chalk it up to ethical issues, planned suppression by Section 31, Discovery having jumped out its original universe, or something of that sort, because, although it's been all too frequent, it's lazy writing to just use technology as a MacGuffin and then abandon it (hell, it's half the reason Threshold is so mocked).
I think your third point pretty much covers it. There is no plan. It's make it up as we go along and if it fits - fine and if it doesn't fit - then we'll just say it fits. The spore, shroom, drive is probably the silliest part of the whole package. Stamets with his arm all stretched out getting plugged in and a bunch of spores added to the mix to bounce them here and bounce them there and it would seem to bounce them kind of... nowhere. It's not like they didn't get a clue from Ripper that being interfaced into the drive system wasn't going to mess with the brain of said conduit. The fact that the spore drive can be random or just crap you out in the middle of who knows where doesn't actually speak for it being the kind of technology that is worth keeping.
I'm not sure what the issue is, maybe they do have a solution and Harberts just didn't want to give it a way. Now that Stamets is a white-eyed crazy god person and Discovery got dumped into a another universe, obviously shit is going to go down with the Drive. It's clearly too dangerous to use which i think is the likely explanation for it being mothballed. Or it could be that they stop using the spore drive because it can only work with a biological computer that is currently beyond federation technological abilities. Maybe the Bio-neural gel packs seen on voyager are a step towards creating a biological computer? Either way, we're only nine episodes in. I don't think there is any need to start clutching our pearls just yet.
That's not a great example, because it was basically just the Byzantine formula for an incendiary weapon that was lost. The concept survived, and other variations were in use during the same time period and afterward (albeit with different components) by the Arabs and Chinese. Variants of Greek Fire were used by both sides during the English Civil War as well.
It would serve them right if they used it this 'one last time' and got stuck in a shitty part of the universe with a redundant piece of technology trickery. Seriously these guys are scientists and they've been conducting the spore drive technology by the seat of their pants, which given the status of the war they could excuse. Stamets was so busy worrying about compromising a loved one that he hid is condition. Why wasn't it monitored better? Interface upgrades were their priority. Yet after Stamets extended himself with all those crazy jumps his idiot of a Captain gave him the go ahead to do one more jump and probably did a bit of re-setting the destination himself. That is hardly clever thinking.
So what do we know about the Spore Drive and how it works and why it works? It need a specific kind of spore from a specific fungus to function. There begins a potential supply problem. The drive can only be used at short ranges without a navigator. Doing otherwise risks jumping into a sun or worse. We are talking jump distances of like to the Moon and back...hardly worth exploration travel for a warp powered starship. The original navigator entered USS Glenn by some chance. Its origins are unknown, but its DNA match is closest to humans in terms of intelligence species. Finding another tardigrade would be difficult, as USS Glenn got one in their storage compartments, while USS Discovery, which was growing the stuff, didn't get one. The original navigator seemed to be a limited use creature and it basically failed via almost going into hibernation. That's not all that practical. The DNA extract from the tardigrade might be a limited supply item, so it is unclear if another ship's navigator can even be created. The only species found to be compatible with the tardigrade DNA was human, which have laws against preforming such DNA augmentations. The augmentation seems to adversely effect the human navigator over time...and not all that much time, since the ships only been jumping for a few months at the most. The Spore Drive has serious dimensional hopping properties, and could have been jumping timelines the entire time, just they timelines they were jumping too were so close to their own that no one noticed. One of the Drive's inventors and the founders of the principles behind how it works, is dead. The other one is started to go nuts after being augmented to be the ship's navigator. While Starfleet can probably build more Spore Drives...can they make more navigators? The Spore Drive, as presented has several flaws that would need to be worked out for any other species in the known galaxy to even use it. It requires a specific fungus to function. It requires a navigator to be of any practical use outside short jumps within a planetary system. That navigator needs to be a human or a tardigrade.....good luck finding a tardigrade. The DNA extracted to augment a human into being a navigator might not be easy to come by, if at all. Especially if USS Discovery has the only samples. It may be unethical to augment more humans to become navigators. And outside of that, it would seem that no other race is a close enough match to work as a navigator. They could compound this by making the tardigrade a hyper-evolved Stamats...and thus this is a loop, and there is only one navigator, and no one ever figures out how to make it work without Stamats. The solution would be to invent a supercomputer that can handle the calculations for the Spore Drive...but is that possible, or does it require a living creature? As for Genesis....it could be as simple as David Marcus didn't write everything down, because of his use of protomatter, so he wouldn't get caught. Anyone else working on the device would find it doesn't work, because the element needed it not mentioned and slamming in protomatter is unpredictable and unstable.