Is there an email client that allows me to access a mail account read only? Which means that cannot change any message, any flag, and the like. It seems that Thunderbird does not provide such a switch, but perhaps other clients do have this functionality.
I would accomplish your requirements by setting up nginx server (even locally) and setup IMAP proxy in it, then configure it to deny certain IMAP command (the ones which modify anything). nginx is not only capable to proxy http, but also IMAP. then point your email client to the newly set up nginx as IMAP server.