Many thanks for the advice and the helpful links. However, over the last two days, I have tried all of these fixes at different points. As soon as one seems to work for one problem, I find myself with another problem. So, for example, this afternoon, after the original launcher hanging for three hours, it suddenly gave me the new launcher. Progress. Now, I have the same problem I had yesterday afternoon: I'm half-way through downloading Main Assets 1, with streaming disabled, and I keep getting the "unknown error while patching". This is two days of problems, all the while my pc out of commission because I'm trying to install a game which ran perfectly until a couple days ago.
One begins to wonder whether it's all worth it.
But - sincerely - thanks for the reply and the links.