"Harmonics" generally refer to the barrel itself. Barrel resonation, the shock wave that travels from one end of the barrel to the other at supersonic speeds. If you're familiar with OCW (optimal charge weight) load development process, this seeks a "node" where the muzzle is at an optimal location when the bullet exits the bore as the barrel whips due to this shock wave.
(Stress free) bedding is a different issue, but a critical component in building an accurate- as in REPEATABLE, rifle.
If the action moves in the stock under recoil, and does not return to EXACTLY the same position, this will affect accuracy. It's not related to barrel harmonics, but just like the barrel itself (most important, IMO) a critical factor in what makes for an accurate rifle.
Consistent accuracy, is nothing more than consistent repeatability- getting the rifle system, to do exactly the same thing, every time.
Thread fit of barrel to receiver is far from critical, and I'll take a slightly loose thread fit over one that's too tight every day. Especially in the realm of production actions, the threads will not be precisely perpendicular to the receiver ring- this is why we single-point these (true) the actions.
If the thread fitment is too tight, there will not be enough "play" in the crests/roots to allow the barrel shoulder to mate up precisely to the receiver ring (these two surfaces MUST be true/perpendicular to the bore and raceway).
Threads bring the parts together. The shoulders/mating surfaces align them.