Hey Michael.

Clearly I'm not intimately familiar with the ins and outs of implimenting an idea like welding endpoints together, so you'll have to forgive the idea's bluesky nature. (Clients of mine always ask for the impossible out of ignorance, so why should I be any different!)

I was imagining a tool much like join (and you're right, unweld as well) which, at the line/curve level might show a dot or highlight around the welded endpoint(s), letting you know something is going on there.

As to it's behavior, it would make sense to me to be able to move any points including the welded point independent of the other points, but not the line/curve as a whole. I imagine you could transform the line in any way as long as the welded point was the anchor and did not move. For instance, rotate around the welded point, stretch/lengthen from the welded point etc.

Now that I'm writing this I'm reminded of Illustrator and how it differentiates between independant lines/curves and continuous (after independant lines/curves are joined at their endpoints) lines/curves. The lines/curves and points are fully editable in either case and only the lines/curves behavior tells you if they are a series of lines/curves or a continuous line/curve. Illustrator also has snapping, which I was greatful to find in MoI, as it makes life SOOO much easier.

Not certain I understand the issue with the trim or boolean operations. Once a welded endpoint is trimmed off or removed due to a boolean, wouldn't it just be deleted from the resultant line?

Anyway, just food for thought.