Pretty much this, except if a transplantee is able to unlock further evolutions himself, then he can obviously revert back after that point; ie Kakashi. A transplantee like Kakashi could theoretically unlock Rinnegan given the right DNA and both MS. If he did, he might be able to use all the jutsu.
Right, it would have been bad writing to do so. It only makes sense that someone who is transplanted the final form of a doujutsu cannot regress it. However, if they start with a less progressed doujutsu and evolve it themselves, they'd likely have access to everything they unlocked along the way. It might have been convenient for the plot for it to work out this way, but it was the proper way to do it.