I'm just going to focus on one piece of this. Ultimately the SLDF relied heavily on orbital bombardment to win in some of the toughest battles/campaigns and theaters of the Reunification War. When they hit real obstacles, such as Taurian worlds, they used orbital bombardment and WMDs on their cities, militaries and everything else.
What would some Level 2 prototype tech have been able to do against that?
Not much. If anything a tougher fight, would have meant more of that in more places as far as I can tell. On many worlds the SLDF stuck to conventional warfare if they felt like their forces could handle it. They resorted to the WMDs when they felt like their forces could not handle it. Apply that same logic, and you just see a lot more worlds getting bombarded from orbit.
One of the key tenants of that era is just how lopsided it was. It wasn't a 55/45 split, where shifting one impactful variable might have truly changed the dynamic. The gap between the SLDF/combined Great House forces and their periphery opponents was just far more dramatic than that.
The best case scenario for the periphery nations really, was making it so tough for the Star League that they gave up, they'd quit. They would lose public support and lose the desire to keep fighting and instead start negotiating an end to the war. That was especially true in space among the navies, where the SLDF was just so overwelming that the best the Taurian navy could hope for was to create some specific situations where they could gain local superiority in a specific time and place for a brief period. However in spite of how bad it got; the Star League never seemed to really falter. Sometimes SLDF leaders got replaced, sometimes there were criticism and a rethink of tactics and strategy, but they never really lost the will to keep the war going.
I don't think the periphery nations having some Level 2 tech would have changed that very much. One of the justifications for the war was the idea that the periphery nations were a threat, particularly after the Taurians thrashed the Davions around a bit. Giving them better technology just makes that case stronger, IMO. While not ultimately changing the outcome.