Of the folks using them who may have ridden in the wet and washed your bikes, how well sealed is this from the top or is the tool likely to rust over time?
There’s an O ring at the top of the tool to help it stay in place and seal it from the elephants, and a lightweight ‘bung’ under the steerer that stops the tool from rattling so it’s pretty well sealed.
Re. the cutting of threads, I’ve installed a few of these, including the one on my bike and there’s so few threads that I’d say it’s a non-issue. Even if you were running no spacer above the stem, the threaded section would probably only just reach the upper stem bolt. Someone commented on steerers being some precise engineered product which they are not. They are a ‘one size’ fits all / cut to length bit of pipe in simple terms. If they were that precise, they’d be size specific, with butted / tapered internal profiles related to the length of the tube and probably even be tapered to a thinner wall right at the top where there are lower leverage forces, just the clamping forces (generally only 5Nm) from the upper stem bolt.