One good lesson through all of this is:
If you can not nail down exact problem, start replacing cheapest item
first within system where problem exists.

This is an order to consider for diagnosis:

Check for white exhaust steam
Check oil color if milky may be internal leak
Check coolant level when hot- be careful of hot/high pressure
Check coolant color- should be clear yellow tanish cast not brown or
redish (transmission leak) or green(wrong coolant), MB coolant is
special mix (or Zerez GO5)
Check for leaks- everywhere
$6 radiator cap---- just replace
$175 ea Electric Fans---B4 you buy- make sure they turn on when hot-
if lazy to start after tapping (see prior message for tip to resurrect
fan motor)
Free Pressure Test--- Autozone test equipment loaner
$40 thermostat----B4 you buy--can remove and try the hot water test to
open/close it (see prior message for tip removing)
$250 Radiator----B4 you buy--try flushing it with hot water and feel
heat transmission to see if clogged or remove and take to radiator
shop to clean/test it ($80)
$375 Water pump---B4 you buy--try all prior steps
$400 Fan & clutch---B4 you buy-see how is spins when hot and cool (may
not operate well is other system problems)

if you have a bad head gasket it might show up during pressure test
(not sure about leak fixes with pour in products.. ugh)

I'm sure there are a couple of other nuggets to check cooling system
but also make sure you have good coolant and proper amount bled
thoroughly.

Good luck