I feel the rules are pretty clear about this, so I will just quote the rulebook:
"Overrun: Unlike other units, a tank may move through (but
never remain in) a hex containing enemy units (even through a
hex that is at the stacking limit). The movement cost for an
overrun is the normal cost to enter the hex plus one movement
point for each enemy unit in the hex. Immediately after the
active tank leaves such an overrun hex, every enemy squad in
the hex immediately becomes pinned (vehicles are unaffected, as
are squads being transported in vehicles or located in entrenchments/
pillboxes). Squads already pinned or disrupted are unaffected.
A tank may never end its movement in a hex containing
However "entrenchments/pillboxes" should now say "entrenchments/pillboxes/trenches" due to the new fortification type added by the FotB expansion.
1) units = squads, vehicles (tanks, trucks, half-tracks), anti-tank guns. Counting all units in a hex including those in a fortification (entrenchments/pillboxes/trenches*)
2) only squads get pinned unless they are in entrenchments/pillboxes/trenches
3) squads already pinned or disrupted are unaffected. (clearly stated in rules)
4) Panzerjaeger is correct. Tanks can enter the hex with entrenchments/pillboxes/trenches, but cannot enter the entrenchment/pillbox/trench fortification itself. So a tank could overrun a hex with entrenchments/pillboxes/trenches in it, but couldn't occupy the trench itself and take advantage of its cover.
Example: if a tank wants to overrun a clear terrain hex containing 1x tank, 1x truck, 1x squad that is in an entrenchment. The movement cost to overrun that hex would be +1 for the terrain and +3 for the three units in the hex for a total of 4 movement points. So it would need a minimum of 5 movement points to pass through the hex as it can never remain in that hex. If it doesn't have the 5 movement points, then it can't attempt it. Immediately after the tank leaves the hex, nothing is pinned.