You’re missing the point:
West-Ukranian neo-nazi != Ukraine
If Russia intervenes, it would be on behalf and at the request of Ukrainian east and south, and would likely have most of Ukrainian military on it’s side. So the op would be largely a mop-up of the west. Now guerrilla warfare is messy business, and would likely take a very long time. However the scale is nowhere near what you described.
p.s. Personally I don’t think any of this would happen. Ukrainian east and south has all of the industry/finances of the country. The whole conflict will be decided on the economic battleground.
