Smart.
I don't know. I do almost exclusively Subaru but I have done a couple other manufacturers head jobs. There's two heads and they're horizontal so they're not easy/veritically mounted right on top like some and there's twice as many as a vehicle with one head. But there are no special tools required and nothing is tricky like some vehicles have impossible to access bolt/cranks due to cramming those transverse mounted engines against strut towers. So it depends what you're comparing it too.
Most people pull the engine, it's tricky to do in the engine compartment but can be done if you unbolt the motor mounts (2 14mm nuts - easy) and jack up each side as you're working on it to tilt it so the head is somewhat "up". Lift drivers side, work on it, lift passengers side and work on it. It's certainly nice to not pull the engine for some people.
Use Subaru gasket, resurface the heads, clean and lubricate and probably torque the head bolts. Install a Subaru timing belt ($70) and lower cogged idler ($30) from Subaru, check the timing tensioner for oil around the seal (if it's wet replace the tensioner) (or just get a complete timing kit with all new pulleys and tensioner). New valve covers are smart while it's apart. Then you're good for another 100k with minimal maintenance. Then add a bottle of Subaru's coolant conditioner - they "require" it. Although I don't think GD even uses it when he does a HG job.
Overflows are old and dirty I don't like to assume to much from them. A rubber seal on a radiator cap can degrade and get flakey and end up with black particulate matter in the overflow...etc.
That's a horrible suggestion. These engines usually don't respond well and can still overheat without a thermostat. Also the coolant flow often gets disturbed and you loose cabin heat anyway when they start overheating because it's not cycling through properly - basically the same thing that makes the thermostat trick not work also makes the cabin heat not work.
Also I've pulled heads that were driven for a couple months with bad headgaskets like that and they will wear the metal on the head down like water wears down rocks. You'll have a smooth, valley of warn metal where the head gasket breech is if you drive it long enough like that. Right by the combustion chamber will be wide and it'll chamfer down to a rounded point.
Avoiding additives and blue devil is a good idea.
The Subaru Coolant Conditioner works 100% of the time on initial overheats of factory installed gaskets....but this isn't an initial overheat and I didn't want to mention it unless we confirmed an external coolant leak - that's the only condition for which this will work. If it's leaking internally it's a waste of time, effort, and coolant.