Coolant is pouring out as fast as I add it?

Yes, that is a threaded gallery plug. If it is leaking that badly it needs to be replaced. You would drain the coolant, then use a proper sized hex wrench to break that one free and remove it. Then you clean the corrosion and crud from the hole. Once clean you apply thread sealant and install the new plug.

I generally use OEM parts when possible.

You apply a thin coating of thread sealant to the threads, then it gets screwed in. Then you let the sealant cure. Then fill it. The interface of the threads with the sealant stops the leaks. If you want to see real oddities look at the conventional disc type freeze plugs. They machine the hole so it's round with a ledge, then you put a tiny amount of 3M sealant on the edge of the plug. Put it in place and use a hammer and punch to compress the dome in and force it to lock into the hole. The cooling system is only subjected to about 20 psi maximum as a worst case, so it really doesn't take a lot to seal it. What commonly happens is the coolant doesn't get changed or people run straight water, and they rust out from the inside.

Yes, that is the process. They rarely fail but just about every failed one I've dealt with was for corrosion.

Yep, lefty loosey, righty tighty. There isn't really a torque spec on them, just in tight so they seat properly.

I generally just grab one that looks right while working. From the image I would think it's between 15 and 17 mm but I don't have one to check. They are not a common replacement item.

Normally I wait at least a couple hours with that.