XMarks Safari constantly re-downloads old bookmarks
I seem to have solved my issue, as it's been quite a while now and the problem has not reappeared.

First I turned off Safari XMarks' option to automatically synchronize, so that it wouldn't mess things up during this process.  Then t had to get my bookmarks in Safari all clean &amp; perfect, and once I had that I manually overwrote the server bookmarks with the copy in Safari.  Since then, my old "bad" bookmarks have not reappeared.  I suppose it's possible that Safari is quietly in the background still downloading and overwriting my local with the server bookmarks from time to time, but since the server copy is now correct, if that's happening, I don't notice.

Everything seems fine now, I can use/update bookmarks in Chrome and Safari and they all stay in sync, though there is a definite lag in time for them to pick up changes made from one another, that's just a fact of the way the software works it seems.