The thing is that most tech are messed up and it is not simply a T-70 issue and I simply do not accept the argument that T-70 has to be "OP", it does not.
There are number of fixes for this:
One can delay 222/FHT by reintroducing 221
One lower the price of OKW T2 to be the same as T2 and move the extra cost to engineer and make a prerequest for T2
And I am not sure why in your opinion the T-70 should be a counter to PzII or FHT, Soviet have zis and where given PTRS to deal with these threats, if these option do not work there is something wrong in that design.
Your solution of making T3 come with a previous building simply makes thing worse as even more powerful unit become available earlier. This is direction that should simply be stopped. When something does not work one should consider reverting back instead of continuing down the same path.
I think the T70 needs to win VS p2 because of the timing of the units. I'm not against having the T70 toned down to P2 levels (though it would overlap with the AA halftrack completely), but that can only work if you reduce the soviet up-front tech cost to actually allow the T70 and P2 to arrive at the same time. Simply downscaling the T70 is equal to massively nerfing the faction as a whole, even though the cost/performance ratio of the one unit you are changing remains the same.