Some additional comments...
Firstly, my CSS problems were a caching artefact and not a genuine problem after all... I had turned off caching on MODx and cleared the cache on my browser, but had forgotten that by default all my web requests go through a caching PROXY SERVER; turning this off as well meant that changes to my CSS file were immediately reflected, so that at least the bizarre behaviour stopped.
Secondly, it seems that TinyMCE just DOES add a <TBODY> tag to tables regardless of how you enter them... this is not a problem once you are expecting it... in fact using TBODY, THEAD and TFOOTER makes my particular tables easy to style with no need for artificial classes...
Thirdly, I found a neat site
http://veerle.duoh.com/index.php/blog/comments/a_css_styled_table/
that gives a good description of styling a table with CSS; along with the comments that this blog entry generated, I now have a much better understanding of what is happening with tables... how about this
tbody tr:nth-child(odd) td
as a CSS selector for alternating rows (not that it works in current browsers, mind you) !
Cheers
Gordon