Partial mobilization would have also not worked because NATO( Ukraine is a place where NATO and Russian are fighting) would have still retaliated this time. They all were preparing past 2-3 years. This is why Americans left Afghanistan to do action in Ukraine.
No, Russia would have mobilised and taken over most of the left bank before Ukraine could resist or NATO could interfere. UAF at the time was totally incapable of defeating Russia.
With enough troops, the Russians would have rushed towards all the major cities along the river and UAF troops on the left bank would have gotten themselves surrounded, they would have surrendered within weeks due to lack of supplies. In fact this is what NATO expected would happen. No one expected Russia to only invade with some 80-90k ground forces against nearly a million Ukrainians.
Normally with Russia's forces, Ukraine should have fallen in less than a month. The fighting on the left bank shouldn't even have lasted a week. It's pretty much what we have planned with Pakistan, except we have over 500k troops and twice the tanks and artillery in half that space. Plus an air force that we will actually use, against an enemy that's not gonna get NATO assistance.
Why else do you think Ukraine was never armed by NATO?
Screwing up this war is completely Russia's fault.
Even now, if they bring in enough troops, they can make a beeline towards all the major cities and this would end the war very quickly, 'cause about 500,000-700,000 troops would get surrounded. It can be done before December ends, before more weapons are supplied to UAF over the winter.