I'm not sure I have all the facts 100% correct, but here goes. 70mm and 35mm have the same aspect ratio. For SW and ROTJ, I believe the 70mm print is visually just a blow-up of the 35mm print. The 70mm version of ESB has some slight visual differences from the 35mm/home video version. All of the 70mm prints have surround sound mixes which the 35mm didn't.
All screenings would pretty much have to be private non-commercial deals. If you live in Southern California, Mike Verta plans to do private screenings of his personal restoration project. I'm considering letting my local hippie independent theatre know about Harmy's reconstructions (I know they do private screenings and have a AVCHD-capable Blu Ray player, so it's no stretch). But seriously, just get into backyard projection. It's fun, cheaper than you'd think, and you get to know your neighbors. Be your own private screening theatre.