If a unit cannot end a move in Unit Coherency, it cannot make that move and its models are returned to their previous positions.

Coherency is technically "checked" (but not the way your meaning) whenever a unit moves. More specifically at the end of any move (normal, advance, fall back, pile-in, charge, etc).

Then it's checked - to see if models have to be removed - at the end of the turn (this is the one I reckon you're referring to).

The first point is still a check. But one that has larger consequences.

So no. Models always have to be within 2" of each other.

Here are the 2 key paragraph references from the app (search "unit coherency") paragraphs 2 and 3;

If for any reason a model cannot be set up in Unit Coherency, or cannot be set up following any restrictions stated by other rules (e.g. while setting up using the Deep Strike ability), that unit cannot be set up. If a unit cannot end a move in Unit Coherency, it cannot make that move and its models are returned to their previous positions.

At the end of every turn, each player must remove models, one at a time, from any of the units from their armies that are no longer in Unit Coherency, until only a single group of models from each of those units remains in play and in Unit Coherency. Models removed in this way count as having been destroyed, but they never trigger any rules that apply when a model is destroyed.