When I talked of the Ukrainian Army folding it was case of Russia going full bore. The frontiers are long, very long. Ukraine has a very long border with Belarus. It is not a given that Russia will come in through Belarus, but how can Ukraine be sure? So Ukraine has no choice but to position forces on her border with Ukraine, just in case. The Donbass frontier with Russia is also long. Then Russia has ample forces in Crimea to launch offensives from there as well.
But I dont see Putin doing that. Another Russian leader may, but I dont see Putin doing that. He has so far always appeared to settle for less than he could have obtained with the forces he had. In Georgia Putin could have pushed much further than just the breakaway portions; there was nothing to stop him. In the Donbass he had the Ukrainians on the ropes. It seemed to me like he settled for far less. He didnt have to seize major Ukrainian cities, but he could have pushed his artillery to within range of say Kharkiv. Put Kharkiv within artillery range, not take it.
I will have to leave it to those who have a better grasp of military matters. I was a cub scout as a kid, that is the closest I ever came to a military uniform .