Thanks RetroTechie, for emailing your back up to me. I went through my backup with Kompozer and replaced the ? with space and it looked good in Echo and Firefox on my Linux and Windows 10. I forgot the easy solution was to change utf8 to western .
RetroTechie's backup is complete and cleaner than mine so I put it in my dropbox and you can get it at https://www.dropbox.com/s/ewop992480t9w ... r.zip?dl=0
I narrowed down the issue to a little MS-DOS program called ROMKEY.EXE (contained in ROMKEY.ZIP), which is used in a ZX81 <-> PC keyboard / data transfer project. Turns out ClamAV virus scanner thinks there's a trojan in it (99,99% sure it's a false positive). On Virustotal.com, ClamAV is the only engine among 60+ who thinks so. Anyway I suspect that ClamAV's flagging might be the reason for the Gmail troubles.
Due to how annoyed I was about Gmail's flat-out REFUSAL to send this archive (not just a warning, but simply no-go even if you know what you're doing ) with no easy workaround, and the fact that others may hit the same problem, I decided to report this false positive & upload this ROMKEY.EXE to ClamAV website.
Let's hope they resolve this - stay tuned!