I doubt the US would build a launch site in Kourou even in the remote chance that the French allowed it. Not only would ESA not welcome the competition, I doubt the US would want to be dependent on the French.

It takes a lot to get an operational launch complex. When the Air Force planned on launching Shuttles into polar orbits out of Vandenberg AFB, they spent about $6 billion on the facilities. Those were in 1980s dollars, so the price would probably be around $10 billion today. Not only are you talking about a launch pad, you need all sorts of payload and vehicle processing facilities. This stuff isn't cheap. Some of the costs were driven by the unique requirements of launching Shuttles, but a lot of it would be common with any manned system.

I don't know if Hawaii would be a good site. You have to launch towards the east, so you have to make sure that rocket stages or falling debris won't hit people on the ground. You might be able to safely launch from Hawaii, but you'll still have the problem of building all of those facilities. Only, in Hawaii they'd cost more. It's an expensive place.

I saw a proposal this morning to move the ISS to a lower inclination using magnetic tethers. The idea would probably work but it could take up to 4 years to complete the job. Once the inclination dropped below 51.5 degrees, you'd no longer be able to launch anything from Balkinour (Tyuratam). You'd either have to launch from the Cape, Kourou, or perhaps one of the other countries like Japan, China, or even India. Since we're so dependent on the Russians for ISS support, you wouldn't want to start reducing the inclination until their new facilities are operational in Kourou. That's going to take several years. I've read they're planning on launching Soyuz boosters from there, but I don't think they're talking about manned Soyuz missions.