Excuse me, but according to you the only way to conquer Moscow for the German is by mean of a massive blitz of panzer? One have to use a “can opener” move to win? It is not possible to conquer Moscow in other way?
Then use Italian forces: attack the territory during Italian turn, conquer the territory, conquer the AA and then blitz all the panzers across the territory.
AA is a unit. No matter if it roll dices or not it requires a battle for being captured, this is acceptable for me. Also defenceless transports require a battle to be destroyed even if there is no one die rolled. Also this is acceptable to me. Moreover it is simple to be explained and it is easy to be remembered.
Gaming the system? No I believe it is called “keep it simple strategy” (KISS). Making a rule and then introducing several exceptions is the way to create a complex ruleset that may cause problems to new players and quarrels between experienced player when one of them did not remember Exception 1.3 “Blitzing through ostile territory with only an AA Gun present and no other ground unit” to Rule 4.17/A “Moving tanks of two spaces during Combat Move”. Moreover, the more are the rules, the greater is the possibility of errors, misprints, and other problems in the rulebook that have to be corrected, with FAQ, Errata etc.
Considering your solid russian strategy, I have some doubt.
Consider a stack of russian unit in Belorussia and an AA Gun in Eastern Ukraine. Consider a German Stack in Poland.
If German is strong enough to move its stack in Eastern Poland and Russian Army is not strong enough to destroy it or at least strafe it, then your strategy is in problem. Because now the Russian army is in a dead zone created by the German stack present in Eastern Poland. Next turn Russia have to make a choice: stay and die or run to live. If they select to stay, next turn German player will attack Belorussia, killing the Russian Army and preparing to invade Moscow, while other panzers and/or fighters in NCM move to Eastern Poland. Panzer will blitz across Belorussia next turn.
Otherwise German army may move in Eastern Ukraine, capture the AA Gun, using it to cover its advanced army.
In your scenario, moreover it is possible also to do the move that you would like to do. If Germans attack Eastern Ukraine and capture the AA Gun then the Russians have to counter during russian turn. Problem: AA Gun, being involved in combat, even if they do not roll dice, may not be moved. So the Russian have the problem to cover the territory where the attack come from: Belorussia. AA Gun cannot be moved to Belorussia, being involved in combat. If Russians leave only an inf there… then the can opener move may be performed: using Japanese airpower to clear the blitz path for the panzers.
If Germans are not able to deadzone the territory in wich Russian army is placed then they do not deserve to conquer Moscow.
What I am trying to say is that A&A is a game that requires sounding strategies. Tricks and expedients may work in few games against some players but there is no solid strategies based only on “gaming the system” approach.
Said this, you may use and propose a House Rule in which AA Gun are not able to stop the blitz, every one is in his right to play the game in the way he enjoys more. So if you feel that the rule is wrong ignore it or make an exception to it. Everyone that likes that HR may use it.
However, there is no need to try to change the OOB rules, because other player, like me, may enjoy to play the game with the rule as they are.