Can I go to Mikveh on “Erev Shabbos” after Shkiah if I came back late from Mivtzoim?

Technically it’s OK, making sure to abide by all “Mikvah on Shabbos” laws:

However, one will be lacking the significance of going on Erev Shabbos as a preparation for Shabbos, which is the main aspect of Mikvah then.

Of course, one should not miss Mincha with a Minyan and at the proper time because of that.