Slitherine

If a LF unit tries to pass through say a pike column. Even with the extra 2MUs it cannot make it. Hence it is left in the 'middle' with some bases behind the pike and others in front. Does this unit have to complete the interpenetraion going the same way until it finished the interpenetration OR can it turn 180 degress and move normally back the way it has come? If the answer is the latter where do you measure from?

The rule states that:

When a partial interpenetration has occurred, no wheel or turn by either battle group is permitted
(except a 180 degree turn by skirmishers) until either battle group moves clear of the other using a permitted interpenetration.

Therefore the skirmishers can turn 180 deg before moving. If they do so, their move is from their new front edge, and the bases that must interpenetrate to 'catch up' may add 2MUs to their move. (and the BG loses its shooting ability it they do)

additional rules:

A battle group completing a previous partial interpenetration reforms at the end of its move.
It reforms into normal formation facing the direction previously faced by any of its bases (player's choice) and level with the furthest forward base in that direction.

Which means that if it moves at all, it MUST end in normal formation (i.e. it cannot make another partial interpenetration)