If you look at the Attack Victim event in Unit_Parent you can find the nodes I use for getting the attacker to face its target. If you duplicate these nodes and have them affect the target as well, the target should automatically face the attacker.
A simple, though not very efficient way to get all units to face the closest unit is to enable tick events on all units, have them find all actors of class Unit_parent each tick, find the distance to each and face the one closest using the nodes I mentioned. It is generally not recommended to run through arrays on each tick for multiple actors, though. A better way could be to only run this code while a unit is moving, and only comparing the distance to the current moving unit with the distance to the previously closest unit (which could be stored in each unit). Something like that should work well and not be too inefficient.