1997 Subaru Outback overheating

Hello!
My Subaru is overheating, but only after i drove it for ~20 minutes. Can idle
forever without overheating.
So after i drive it those 20 minutes, and the temperature gauge goes all the
way up to hot, every time, my engine coolant overflow tank, is full and
actually, overflowing.
If I continue to drive the car (which i only did once to test this), the coolant
keeps overflowing.
If I turn of the engine and wait 4-5 hours, the overflow tank, is always
completely empty, and there is never any coolant lost on the ground or such.
It just magically disappears.

I do not have white smoke coming out of the exhaust at any point before,
during or after the temp gauge goes to hot.
I do not have milky oil, and no oil or coolant leaks under the car where the
heads connects.
Bubbles are present in the overflow tank
No smoke inside the engine bay.
Cooling fluid color is green (even though i believe it should be red)
I have changed the thermostat myself. (same symptoms before and after the
change)
I have tried the same as described above (driving until overheating) with a
different radiator cap (from another 1998 Outback i have which does not
overheat) - same symptoms.
The radiator fans seem to turn on and off at the correct times.
Engine bay is hot, sometimes very hot.
Occasionally the radiator top part is extremely hot.
There is no cold blowing from the A/C
I live in a colder country (max temperature 28 Celsius)
Which brings me to my 4 questions:
1- Why is the coolant gathering in the overflow tank beyond the point of
spilling out? Why doesn't it go back into the radiator/system when the car is
running/overheating?
2- Where does the coolant go after the 3 hours of waiting with engine turned
off?
3- If the "bubbles" in the overflow tank are exhaust gasses, is that a 100%
guaranteed that the head gasket is blown, or can there be exhaust gasses
there from a bad radiator/other part of the cooling system?

I would also like to add a follow up question, just in case your answer to nr.2
will be "back into the radiator", and that is - how can a full overflow tank, go
back into the radiator, when the car is not running? What is pulling it back?