How can I make a trigger that doesn't fire its actions until all the remaining units of a player have entered into a region or within the boundaries of a point. I also need to make the units move once all the remaining units enter the region/point.

Another question: What would a trigger look like that checks if a player has any units on the map or in a region/point?

Just make the trigger either not run if conditions haven't been met or use "Wait for Condition" as the first action for the trigger.

You can use the "Find Units" condition to check for a number of units within a range of a point.

One way of going about it would be to use a unit group variable and add the players units to this group once they reach a certain point/area. If they leave the certain point/area, remove them from the group.
Then you can have another trigger firing either periodically or once every time a unit reaches the point/area - that checks if all of the units controlled by a player are in the unit group. If they are, then commence with the actions.