1) I checked the revision history and uploaded versions, and there had not been any additional conditions (perhaps you modified your local copy?) - since there is no quick way to compare sprite state in GM, forcing caching would cause side effects if source content is modified. If you need to change cursors very often, you might instead want to rearrange scripts to pump out a cursor buffer and then take such buffer.
2) There is a source canvas somewhere inside the the HTML5 texture object, but it is a few layers deep. I usually avoid implementing extensive hacks in smaller extensions.
3) That would seem correct, I'll fix that for the next release