The key thing now is whether you're within 1" of an enemy unit/model, and whether you can move within 1". There's no such thing as locked in combat any more, just whether you're within 1" of an enemy unit/model or not that then affects other rules.
The movement phase has a rule saying you cannot move within 1" of an enemy model in the movement phase. The charge and fight phases do not have a rule like this, and that rule specifically only applies to the movement phase. So you can move within 1" of enemy models for pile-in and consolidate moves in the fight phase. If you're already in base-to-base, you cannot move any further. Overwatch happens in response to declared charges, not pile ins or consolidate moves.
A successful charge requires your first model to get within 1" of one of the units you declared a charge against. You cannot move within 1" of a unit you didn't declare a charge against, and do not have to move base-to-base or engage as many as you can. Obviously all those you declared a charge against got to fire overwatch if they didn't already have someone within 1".
In the fight phase, any unit that has charged or currently has a model within 1" of an enemy unit can be picked to fight (with chargers going first). When picked, your unit can then pile in up to 3" - which does not constitute a charge - which can get you within 1" of a new unit, as long as individual models end up closer to the nearest enemy model.
If that unit did charge, it can only direct attacks against units it declared charges against in the previous phase (that are now within 1"), regardless if your pile-in or opponent's heroic intervention has brought them within 1".
Units that did not charge, but do pile in when picked that get within 1" of an new enemy unit subject to the restriction of getting closer to the nearest enemy, can direct attacks against this new unit, as well as the unit that was already within 1" that allowed them to be picked in the first place - subject to individual model placement as laid out in the rules.
One you've resolved the remaining steps of the fight, you can consolidate your models up to 3". Again, there's no restriction on getting within 1" of enemy models, as long as your model ends the consolidate move closer to the nearest enemy model.
Since your fight phase for that unit is now over, and most units can only be picked to fight once, consolidating within 1" of an enemy unit that has not yet fought, that enemy unit can now be picked to attack you in the fight phase, but you can't attack them.
This has its uses. For the cost of weathering potentially weak melee attacks, that enemy unit can either stay and fight again in their next turn, or fall back. If it was a primarily shooting unit, that may harm your opponents shooting next turn. If they don't fall back, then your melee unit is mostly immune to shooting next turn (bar that units pistols).
For fast melee units such as nids, messing up your opponents gunline is a very useful tactic, and engaging additional units in the pile in or consolidate phase - and avoiding overwatch - even if you don't get to attack them can mess up your opponents plans. You only need to get one model within 1" to do that.
In addition to the rules, read the '
stepping into a new edition' and '
designer's commentary' - they help clarify some extra bits.