Right, so you don't need to be a member of the group 'nopasswdlogin' for automatic login. As shared, automatic login you can enable in the Login Window application. Everything else for the user 'm' looks fine and just as it should be.
As I understand you, running any command with sudo from the terminal as user 'm' asks for the password of user 'm' and you can't recall that password. You can reset your password from recovery mode. See here how to do that: viewtopic.php?f=90&t=110928&p=621487#p621487
I suggest you change the passwords for user m and for root (to change the password for root, just run the command "passwd" without arguments). After installation of Linux Mint, the password that was set for the user m was also set as the password for root. Because you can't recall the password for m, I imagine you can also not recall the password for root. So hence my suggestion to change both passwords.
I think you should also remove user m from the group nopasswdlogin, and just use Login Window to set the automatic login. Unless you have replaced MDM with something else, that will work fine. What guide did you follow that had you add user m to this group? You can remove the user from this group also on the recovery mode, with:
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gpasswd -d m nopasswdlogin