Hello again gents, pleased to announce that my plan worked! I used a line of high temp gasket maker in the cover groove, then I gently placed the rubber gasket in the straight areas of the groove. I let it sit for a minute or two and then applied gasket maker to that curve you see at the top there, then pressed the last part of the gasket in and held it there. First time in a while that I felt clever as it took no more than 5 minutes to get the whole gasket fitted snug to the groove.
I haven't run it yet because I want to leave the gasket to dry for 48hrs before I go jiggling it around the frame & engine and possibly wasting my time or causing another leak. I'm confident this will be airtight though as the purpose of that groove is to provide a seat for the gasket, which can then be squeezed by the cover bolts to seal off the valves for good, now I've fixed my threads and gotten that gasket seated, there should be no further hiccups.
I'll definitely be using that gasket maker again for helping my gaskets adhere to surfaces, it seems popular on the YouTube to use it alone but I'd rather get it done the right way. Hope this thread helps someone, thank you everybody! I might post an update after a while.