I've had my GH4 around two years and never have I had a problem with handheld stabilization. Even when using non-native lenses with a cheap Chinese adapter it was very steady footage.
In regard to C4K vs UHD down scaling I would imagine it would be too close to notice.
I'm surprised SyFy didn't eat this up. It looks like something they'd jump on.
It's personally not something I'd watch myself but I know my da would be mad into it. Have you tried going to 'Cons' and pimping it around?
I would go for the G7. I use a GH4 and from reviews I've seen, the G7 is not too far behind.
I would say another reason people didn't like the video moving left is because it was flipped. Subconsciously you would realize it's flipped and it would make you uneasy.
The best advice I could offer would be get out and shoot. Read everything you can find on the internet about cameras, lights, framing, set design. Watch videos from established DP's and lighting people. (This video here really helped me with lighting: https://vimeo.com/33672808)
After you read and watching everything go out and try these techniques with whatever you have available. There's probably up and coming models in your area that would be interested in sitting around while you try techniques out as they get a bit of footage or screenshots for their portfolios.
Without seeing the contract it would be tough to say, but if he signed over the usage and distribution rights you would just need to pay him the agreed fee in the contract and/or the agreed percentage of sales and distribution if there was one agreed.
I regard to the edit, usually the director would have to sign off on the final edit, but if this isn't in the contract it would leave it open to your discretion.
NOTE: I am not a legal professional, but I have dealt with similar scenarios in the past. Also, note: I'm from Ireland so the laws could be different here.