In theory. I'm not sure if Protonmail or the VPN provider would respond to a subpoena, but I wouldn't rule it out. But realistically most of these people are using their home IP address and their gmail account.

There are VPNs not bound by US law and the same with email providers (if your email address with Reddit is even legitimate to begin with).

Yes, but it can't be more than a few percent of people who go through the effort to set that up right? I know it's not hard to do (I've done it), but I think most people on reddit are using either their personal email or a throwaway gmail account they use from their home IP.

You actually still don't need an email address to create a Reddit account.

Reddit has stayed true to their word of maintaining the old design, and it's free of the dark patterns in the redesign. Go to https://old.reddit.com/register and just leave the email field blank or if you use the pop-up menu (the thing you get when you click login/register in the top right of the old design), you can click skip on the first screen where it just asks you for your email.