Yes, your comments regarding os x and Quark 6 are on the money. It also caused a lot of art/design schools to hold off upgrading their mac labs. Only to find, when Quark 6 was finally released that it was impossible to use in a large lab environment (without compromising security or over burdening already overburdened sysadmins) and was crap for a bunch of other reasons. There is a real push to abandon it in favor of InDesign. About time.
OP: I also agree that, while designing for your audience is valid, especially for a portfolio site that is about multimedia, by creating a site that may force a first-time user to download a new plug-in to view, you may just lose them completely. Some alternative content should always be available and accessible even if it just convinces a user to install the darn plug-in.
OP: Regarding the IE 5 internal weirdness. IE 5.1 for the mac seems to have caching bugs. Make sure you kill any cookies/sessions from your site and reload to make sure you are seeing what others are seeing when troubleshooting this browser.