can you recable a grado sr80i with 75 ohm cable?

Will work fine generally speaking. The impedance is specified for high frequencies around 1MHz to 6+ GHz, depending on what type of HF cable it is. At audio range and at these cable lengths this is of no concern. This is just so they have a flat reactance, it's many other factors than series resistance, like how conducting the insulation is and it's thickness. Capacitance is usually good, but there are some types of coaxials that are quite high in capacitance. Series resistance is fine as long as the core is thicker than awg33 and/or is shorter than 2 m, I'd say. What might be a concern though is materials chosen. Don't quote me on this, but I think you want to prefer a pure copper braid shield (with or without silver plating) for audio rather than aluminum foil. I suppose the only really parameter of concern is how flexible the cable is, choose a 7 stranded core over a solid one for ergonomics, and I wouldn't use such thick cables that you usually find with rca connectors to enter the headphone cups. Take a look at miniature coaxes like rg316 or rg178. They can be found in tan/transparent FEP jacket which is kinda sexy.

Not the greatest idea. Coax typically has a thick dielectric to separate the center conductor and the outer shield, leading to the 75ohm rating. This will make it stiff, annoying, and in the case of the Grados, if it isn't secured properly, it would probably tear the solder pads off the drivers.

You're much better off using Mogami / Canare starquad (24 gauge), or Cardas 4x24 if you're looking to spend some money, and remove about 18" of the jacket and run two wires to each headphone.

Also be aware that Grado drivers are sensitive, you can fry them if you heat them too long with your iron.

you can use 110 ohm twisted pair AES/EBU cable, although something totally different is probably better.

Check out Markertek, Redco, and Fullcompass to name a few.