Like you, I don't play parties with much use for Excalibur, so I forgot about that. I generally don't have the inclination to multiloot and I like the benefit of having Igraine, Morgan and Guinevere as soon as possible in the game, so I never really end up with two of anything - it was more if there were any others in fixed locations. It seems this was more a fixture of later M&Ms.
With Windows 10, it does seem somewhat random: I've seen posts here about having simply tweaked compatability settings and getting these games up & running, but no such luck for me. It's the copy protection on the CD versions of games from that era that causes the problem, apparently: it creates a security vulnerability in Windows that was "fixed" with an update in Windows 8, and this update broke these games, but at least you could uninstall the update if you chose to live with the security flaw. With Windows 10, this "fix" is baked into the OS - Windows is completely aware of the issue but made a conscious decision to remove the security flaw even if it broke games. People like us still playing these older games are a diminishing population, unfortunately!
I can confirm however at worst if you're similarly unlucky with compatability settings that the GOG versions of both Wizardry 8 (after downloading the separate "fix the GOG version of Wizardry 8 for Windows 10" patch) and M&M6 work perfectly. The only thing, like I say, is that you're stuck with whatever patch/version it is, because none of the old patches that we have all had for years will work on the GOG version - it's a completely different exe file. I can also confirm that DOSBox works just as well under Windows 10 - I downloaded Pool of Radiance and played it completely through under DOSBox with no issues.
I can't speak for the rest of the GOG versions of the M&M series yet (I know you've a soft spot for M&M3 in particular), but no reason to think they wouldn't also work as well. BTW, the GOG version of M&M6 is the one that comes with M&M1-5 - it's essentially the soft copy version of the Millennium Edition that was released - and that is only $10 currently. The $20 is for the M&M6 package plus M&M7, 8 and 9. So if you don't care about 7, 8 or 9 you can get 1-6 for just $10