1. It's possible, yes. You just have to specify the whole lot, e.g.

Workbooks("My Book").Sheets("The Sheet").Range(MyRange).Value =
MyForm.Textbox1.Text

or similar.

2. Not sure - can you be more specific about what/how is
loading/calculating

3. Just insert:

NameOfUserform.Hide

at the point you want it to vanish. You mention a msgbox, so I guess
the outcome of the msgbox will determine where you put this code.

4. Don't understand the question - if you mean can it appear whilst
using different sheets, then yes.