Agreed: thread drift is a problem, and one I've pointed out before (and as a problem is common enough in community forums to not really need pointing out). That's one of the reasons I suggested Marijke break her original thread into better enable development of the constituent parts. It's also the reason why I started this new thread - rather than drift yet farther from the original post in replying.
As a practical matter, making good choices on forum headings will depend on the supporting software and the time constraints of the site admin. Knowing your plate is full with the degree work, I expect all ideas and suggestion for changing the structure of the forums here will are on hold. You'll let us know when the situation changes; and will provide some of the constraints for the options you see as viable.
In the mean time, a general policy of looking out for thread topic drift should suffice. It's going to happen, that one thought leads to another; and that drift will occur. Like most things, it's only a problem when it gets out of hand. Wonder if a new icon for thread topic drift would be useful. Nothing severe, just a gentle reminder - as in the movie "Lawrence of Arabia" where his friend sees him nodding off on his camel "You were drifting. You've been warned."