I'm sympathetic to the #regex(...) syntax, but you and the proposal both do a good job IMO of outlining why it's probably not desirable. However, I think you've glossed over what I consider the best alternative—one whose benefits are already thoroughly discussed in the proposal: allow for the extended literals #/.../#, ##/.../##, etc., with no bare version.
In addition to allowing unescaped forward slashes (and unescaped /#, /##, etc. at deeper levels of nesting), I think the use of # offers a great indicator of "literal." It's not universal as you note, but it's heavily precedented throughout the language. In addition to #selector we have #colorLiteral, #imageLiteral, #keyPath, #file and friends... I personally have a strong mental association between "#" and "literal" when reading swift.
I still don't believe the proposal does a great job justifying why we need the super-terse bare form to be introduced at the same time as the extended literals.