You have answered Item #1 yourself.
-2. Can I move, add, delete fields/controls
Of course this is a misleading question. The main question is about
Editing a SubForm. Fields are in a table and used in controls. Fields in a table can be moved depending upon the database being used. Generally you can add and delete fields in a table.
As for controls, you can add, move and delete controls on forms, sub forms, sub sub forms, etc. However, I am suspicious you are instead referring to a table control. While you can do much by right clicking on a column heading, copying and moving are bugs in progress - tdf#54021. If I drag and drop a column it is copied. You can then delete the original column. So it works with a bit of a struggle.
-3. My subform is based upon a table. Can I change it easily enough to a query.
Easily is relative to the experience one has. To me this is easy depending upon the sub form. What does the sub form consist of? Individual controls? Many of them? Just a table control? Table control and other control(s). Is there a sub sub form involved? Is the query expected to produce a record that is updatable in the sub form?
For a simple situation and answer - Yes.
Example situation. Sub form which uses only a table control. Delete the table control. Edit the sub form properties and on the
Data tab change from a table to the query you have. Re-link main and sub form. Draw new table control on the sub form choosing what goes where.
For this and your statement:
…as I go along I am learning things wished I’d known at the beginning.
Go through the Base documentation.