I just don't get why it should be so important for Marvin to
immediately show the
ideal font size (when every reader believes something
else to be ideal!) upon the
first opening of a book.

I mean, why do we have all the fantastic formatting buttons in Marvin? Anyone can just push a few buttons, and there it is -- the
ideal layout, and it remains sticky.
As to "MS Word styles", they are notorious for being a disaster. MS Word may be the world champion in producing inefficient, bloated code. I would
not be surprised if any EPUB created in Word, out of all software packages, would fail to display as expected; I would be surprised if it
did.
Does your code get validated in the w3c.org validator, Wyndham? In the EPUBs I create, I avoid anything but absolutely
necessary code, so any MS Word styles are out of the question. I
especially avoid specifying any
font sizes, other than
relative ones for (mostly)
headings, and so, the e-books then look very nice in Marvin not just to me, but to any reader whose reading preferences may be very different from mine.
Specifying font sizes (other than
relative ones) should
not be a matter for the e-book code to handle -- it should be left up to the e-book reader (such as Marvin) to handle, and Marvin handles it beautifully.