Why would 7th Heaven have been abandoned? When Zack named it in Crisis Core, the guy who intended to build it was still in the planning stages, so it seems unlikely he would have built it and then abandoned it so quickly. For me another big question is "what happened to him?" I mean you can do a hand wave and say "he just decided to leave Midgar and gave the bar to Tifa", but here's my headcanon:
Zangan took her to Midgar and left her with Johnny's family. Tifa recovered and needed a job. She has all the qualifications for a good barmaid (huge hooters and a powerful right hook) so the local bar owner hires her. Tifa feels she's really come down in the world - from mayor's daughter to slum bar wench - but she has to eat, right? She can't sponge off Johnny's parents forever. And it's exhausting having to lie to them all the time. Anyway, the bar owner - let's call him Jim - realises she can cook up a storm and promotes her. She's starting to feel a bit better about herself.
Jim is the one who introduces Tifa to Barret. Barret's already living at 7th Heaven with Marlene; he does all the heavy lifting in the bar and odd jobs round the neighbourhood. He seems like a helpful guy and a doting father, with some anger management issues. The bar has a bunch of regulars including Biggs and Wedge and some other OCs. (A bar is the perfect HQ for a terrorist cell: nobody needs to justify their frequent visits to a bar, and every bar has its regulars who are usually a pretty rag-tag bunch anyway; think of "Cheers".) Gradually the knowledge grows on Tifa that they're an anti-Shinra group. The prospect of getting revenge is incredibly revitalising for her. She wants in. And Jim and Barret knows she'd be an asset.
Anyway, Jim gets killed, maybe when an Avalanche mission backfires, maybe in a reactor accident, maybe he's an innocent bystander who gets caught in the crossfire from a Turks shootout, maybe by Don Corneo's extortionists; that's yet to be decided. Tifa and Barret (and their Avalanche cell) have nowhere else to go, so they carry on living at 7th Heaven and running the shop, waiting for an heir to show up and claim the inheritance, but nobody ever does show up, and after a while everybody in the neighbourhood starts thinking of it as Tifa's bar. There's a rumour going round that Jim left the bar to her in his will, that he and Tifa were lovers, but he didn't and they weren't.
Hey, maybe his name is Jim Texas. Or maybe his first name is Texas. Texas McGilray.
That's my headcanon.