AntiMech is just the name of the special physical attack for Infantry units (including battle armor) that are trained to fight against targets bigger than themselves. There's no attaching/swarming like there is in Classic; the models simply stand next to each other and perform their attacks, ending the combat turn exactly as they were left (barring destruction of either unit by turn's end)
1. Mechs (and vehicles) that have not activated yet may move out of close combat, but may not move through non-friendly units. (usually just considered backing away)
2. any unit - infantry or mech or vehicle - that has already finished their movement do not follow/pursue units that back away; their turn has completed.
3. MEL units benefit from increased range (+1" , total 2" for close combat) and increased damage (Size +1) so there's no need to give up these benefits during a Physical Attack. In Base 2 Base, there's only physical (Weapon Attacks are not allowed) so if the unit could move away (between 1" and 2") they could potentially still Melee, if desired, while denying an opponent their AM attack (they now get to use a Weapon Attack, so be mindful of the situation as it develops; lesser of 2 evils is a preference).