Rust - Rust proofing frames, Rusting treatment and prevention

Also, sorry if this is stupid, but should I remove the headset and BB when applying something like this to the inside of the frame and fork, so it's the bare frame with nothing installed? Or just the BB? And if it is applied to the frame without anything installed, what do you need to do to the BB threads and headtube before greasing them, once the products above are applied? Just put the grease on top? I am guessing that won't work.

I don't see any need to remove anything beyond what you absolutely need to remove to get access to the breather holes so if dropping the fork out gets you into the top and downtube breathers, bob's your uncle. If pulling your cranks and seatpost out is easier, go that way. If you have bottle bosses in the downtube and bosses for a tt bag or cable routing or something in the top tube then you can always inject through those.

I guess you need to rotate the frame around to coat all surfaces so the fewer components on the easier that will be but headset and bb cups aren't going to make much of a difference.

I don't do any cleaning of anything before applying grease for assembly, I just wipe of anything that's ran out of breather holes and gotten on the outside of the frame. Mixing waxy oil and grease canny be that bad can it?

Maybe worth mentioning that you'll want to wait until you're sure that anything you've used is all dried up inside the tubes before you put disc pads or rotors anywhere near it. I once built too soon and had Dinitrol run out the seatstay and onto my rotor ehich then contaminated the pads. I have (temporarily) taped over the seat and chainstay breathers at the dropouts to stop any leakage when a speedy build has been required. Just remember to go back and remove the tape once the risk of drippage/leakage has gone.