AT fire in assault hexes

[plloyd1010]

Starting at the end...
 

I think that breaks the mechanic....

It seems to be a presumed mechanic. None of the rules editions I have (2nd, 3rd, 4th & Modern) specify AT fire as combined in action with or as sub-component of assault combat. There is not any option for AT fire for the defender in any version of the rules. The only cross-over is at the end of 4th Ed rule 12.12 where units with AT values may conduct an AT shot for their "free shot".
 

1) if AT Fire is allowed, without ASSAULT---it is not simultaneous----the Russian tanks could take out the PZ without any retaliation---whereas if it
  was considered AT Fire under an ASSAULT it would instigate an ASSAULT fire-fight and the PZ could shot back, either individually or in combined assault.
So allowing AT Fire independent of assault---the dynamic of simultaneous fire is broken...

There is no dynamic of simultaneous AT fire anywhere else in the game. Why do you expect it here?
Of course there is no AT fire by units entering an assault hex. That is precluded by the requirement of rule 12.2 and the restriction of rule 12.3.
 

2) In your example the problem is exacerbated...
  a) The Russians Fire AT while in the Assault square---not subject to any retaliation
  b) The Germans decide to ASSAULT -- PZ either AT fires or combines DF
  c) the Russians (you suggest) get to AT fire AGAIN in their Assault retaliation.....

My hypothetical is extremely unlikely. It assumes that both sides are present in the hex at the start of the turn, and that no one (at least the Russians) has carried over adverse conditions.
In sub-points a & b: That is correct. In b, since AT fire is a separate action, The PzIII might scrab 2 of the T-26's before the infantry attacks.
In sub-point c: I don't see anywhere in the 2nd, 3rd, 4th or Modern rules that you would get an AT shot as part of the defensive side of assault combat. Technically it doesn't seem part of initiating assault combat, but is a separate action, if AT fire is part of the activation.
 

I'm cooped up in 14day quarantine in Bangkok....my opinion also might change in a week when I breath free beach air....

I guess we'll how that is working out in about 11 hours.