I've parked on plenty of inclines with my car just in gear, but I also carry a wheel stop in the car that I use in serious mountain areas like San Diego.
I know plenty of guys that daily with a welded diff. Not my cup of tea (I would never do it) but its possible.
As for the cold start valve. I had a plate cut out of steel and just used rtv as a gasket. For a while I just used a rubber line with a screw in it to block the fuel line, but I did this knowing I would be getting a fuel rail.