First, did it see general combat service in WWII (not as prototype, but as planes introduced to squadron level)
Second was it produced in decent numbers. (figure over 100, although planes have been modeled which had less)
(Snippage).....I assume that large code changes are to be avoided (i.e., "realistic radar" which takes account of mountains would be out due to the code required)............
Exactly! You can wish for it, but the chances getting it in this decade are going to be slim.
I think some planes were put in like you say, "ohhh wouldn't it be great to have ........"
Others might have balance issues, or have been done elsewhere.
To me just because a plane or vehicle is not "late war 44" or wouldn't see general usage in the late war mains is NOT a reason to not ask for it. I think as a general rule more early and mid war planes would be a good thing. Example, when is the last time you saw a D3a1 in late war? Some would say the ultimate in hanger queen. Yet it is a must have for early war carrier actions.
Not to mention that I fly one in the TA almost every day. Its slow speed, large ammo load, and easy handling make it a great trainer plane. Because it slows the fight down, mistakes on merge, or keeping sight are almost instantly apparent. While its ease of flying at the stall edge, or in maneuvers builds confidence in new pilots.
As to why they picked the Firefly, its impossible for us to say.
I suspect they wanted a better allied tank, that they knew the Sherman would be toast for the German ground units.
And so ended up with a compromise solution. But thats just what I think.