Why would a minor nerf to the SU-76 reduce Soviet options? Because right now the SU-76 just dominates everything else Soviets can build because it's incredibly efficient for it's price and it can fulfill multiple rolls.
Soviets SHOULD NOT have the option to refuse to tech up at all; the whole reason behind call in meta being killed was that it allowed people to refuse to invest or risk anything but still get rewarded. OKW can't refuse to tech, USF can only with 1 doctrine, and Ost can't refuse to tech either (again only 1 doctrine really enables no teching) so why should Soviets magically have the privilege of being able to refuse to tech?
The SU-76 just needs a minor change such as a reduction in pen so it can't slap around heavies easily and or a reduction to 50 range to give the SU-85 some sort of reason to be built. I don't see why people are so attached to it outside of bitterness about the days it was once useless.
You seem confused. You keep saying Soviets don't have to tech and yet getting to SU76 requires at least two tech buildings to be completed. You also don't seem to understand that Soviet T4 is now an option not a requirement, but requiring it would reduce commanders that have units that appear in the mid game to disappear since you could not do T4 and use those commanders.
Again, I hate to say this but your first question in your post is directly answered by the first three sentences of the post of mine you are quoting.
Ost can choose which tech to take, but generally it goes T1 T2 T3 (can go T4 in long games). However, some commanders allow for this strat to be modified.
Reducing the range of the SU76 would make it completely useless once T3 hits since a single stug would kill it everytime if it used TWP. Besides it would have to trade damage with Stug on every shot. The reason you get and SU85 right now is because of the vet, focus forward, and deeper health pool. If anything needs to be done the SU85 needs buffs to its accuracy.
The SU76 allows Soviets to play with all of their commanders, that is why we like it where it is. I don't care that it was bad once, I like that more commanders are usable now that it offers reliable AT.
In every game I have seen people complain about SU76 they send tanks straight, in predictable areas, to try to break through and are then surprised that the "terrible" SU76 is able to deal with this threat. Every time I see players using other units intelligently (or even flanking with tanks) the SU76's are useful, but die regularly causing a serious fuel loss and preventing the Soviet player from ever getting to T4 (a serious cost) while also causing them to spend lots of MP to stay on the field. From my own experience I consider them valuable units when I play Soviet, a reasonable threat when I play German requiring some work to bring them down but far from unstoppable. My view is therefore that they are good but not OP.