Many, many moons ago when used party keys I believe I used about 1/4 cup priming sugar for 5 gallons. A 1/4 cup would be 4 Tb so, assuming my memory is correct, 1 Tb would about right for my old brews.
Sealing a party keg shouldn’t be too hard. At least I never ran into problems. A little keg lube can help make things fit easier.
If you try to force carb, which should work, I would do it cold. Crank up the pressure until you hear gas moving and shake the keg until it stops. Don’t put too much pressure in though. I had a couple where the seams bulged out after riding in the back of our van.
Good luck!
Paul