Header Thread Repair

I would say pull the header out and see how much room there is to work. Of course it will depend on which bolt hole it is too, some will be easier than others to get to. You would almost certainly need to use an angle drill and probably a stubby bit to drill it out for the HeliCoil. Once that is done, the tapping and putting in the insert should be easier as those are hand operations.

I believe I'd remove the head and do it on a bench.

I'd probably remove them both maybe have them valve jobbed and have the machine shop fix the holes. Not that I couldn't fix the holes, but if I have to take one off, might as well get some power for your trouble.
Probably cost you like a total of $200.

If it were an old iron head, i'd be a fan of doing whatever it takes to fix it while on the car. An aluminum TFS head? I'd be more careful. And for the record on my old e350 351 work van, I drilled and tapped a head while on the van in a tight spot, held for about 5 years, then while doing it a second time because it was leaking again, yup, I drilled into the water jacket. JB well and a new heli coil fixed it and sold the van (told the buyer, but at $1500 you don't expect much anyway).
I still remember the sinking feeling of the antifreeze coming out.

x2 for dropping it off at the machine shop, I would expect to pay around 300$ to have both heads decked, cleaned and fixed up good as new