You are misinterpreting minimum movement.
Minimum movement rule does not change or limit the Move rating. It has nothing to do with what the unit's Move rating is, or how it's affected by critical hits.
An Atlas with a 6" Move rating is reduced to 3" on the first hit, 1" on the second hit (the minimum Move loss from an MP Hit is 2"), and then is immobile on the third MP hit.
The minimum movement rules says, even if you only have a Move of 1" (Atlas with two MP hits), and the terrain around you is ultra-heavy jungle (+4" movement cost), you can move 2" from where you started. It does nothing to your Move rating, and therefore doesn't affect how critical hits affect your Move rating. When Move is referring to the rating, it should be capitalized. The minimum movement rule is referring to actual movement, not the Move rating.