Craft Commerce 2 — Remove Line Item (and other actions)
Is it as simple as changing the input to a submit (or button submit)
  using the same name tag contents?


Yes.


  Would this then also carry over to be posted via ajax in the same way
  as an update-cart action?


Yes.

Everything can be done on a single line item basis to the update-cart acton, just use the new param names.