That's what I thought too, but apparently it's not that difficult to avoid legal encroachment. Marvel's Superman could fight an alien tyrant, he just couldn't fight an alien tyrant named Brainiac who bottles cities (not until Brainiac himself entered public domain anyway). Lot more leeway than you might expect, I guess.
With so many Superman comics out there you'd probably want a database with names and stuff, just to ensure you don't accidentally come up with the same ridiculous name or gimmick, but that's not a big deal, hell there's fan databases which tracks that already I think. Having to check every script and appearance would definitely be a pain in the ass, and could be a reason for Marvel not to bother with him, but I don't think it's that big a hurdle...more like a minor annoyance.
Marvel does seem to have been successful bringing in public domain figures. Oh, they used Ares for a while too. Not sure how successful we can call him though.If Action Comics#1 Superman comes to Marvel, and you have Superman, his Clark Kent persona, and Lois Lane, you have plenty of stuff to work with.
Lois Lane can become a supporting character inserted with the Daily Bugle or as a friend to Jessica Jones' investigations or Ben Urich and so on. As can Clark Kent of course.
Now in terms of where a PD Superman can fit in Marvel, you have options. In terms of precedent, the obvious one is how Conan the Barbarian has been integrated into Marvel continuity recently. Other ones include Angela from Gaiman's Spawn comics, you also have Marvel's history of working in PD characters like Dracula as a major supervillain and figure who's central to their magic lore. Also Shuma Gorath. And Morgan Le Fay.
I dunno about the Bugle. It's an obvious go-to, the Super cast dealing with JJJ would be great fun, and it'd help integrate the Supers into the MU, but I think it might be more fun if Clark and Lois remained at the Daily Star (the newspaper from Action #1) and that paper was the Bugle's chief rival, or something. Is Parker working at the paper these days? I think Parker and a Golden Age flavored Superman would have a really fun dynamic and I like the idea of Clark and Peter being friends, but work rivals.
I tell you what, I would not envy the person who Marvel hires to introduce their version of Superman. Lots of expectations on that name, and fitting him around the MU....hell of a needle to thread, I think, in a way that could last and not just fade away like so many others have.