You can prevent users from overwriting cells with formulas (or any other cells)
by locking those cells and protecting the worksheet.

Select the cells to lock
format|cells|Protection tab|check Locked.

Select the cells to leave unlocked
format|cells|protection tab|uncheck Locked.

I think that using a couple of cells makes sense.

I've done this kind of thing.

Column A contains the user typed input
Column B contains the "default" value--usually based on some calculation
Column C contains the value that further calculations will use.

=if(a2<>"",a2,b2)

Protecting columns B:C would keep the user from making changes to cells with
formulas.

============
But you could do something like this:

Say your user types answers in column E.
Create a hidden sheet that contains the defaults for all the cells in column E.
(You'll have to update this sheet whenever you change column E on the real
sheet.)

Then plop a button from the Forms toolbar somewhere on that "real" worksheet.

Option Explicit
Sub RevertToOrig()
   Dim resp As Long
   
   resp = MsgBox(Prompt:="Are you sure you want to revert?", Buttons:=vbYesNo)
   
   If resp = vbNo Then
       Exit Sub
   End If
   
   Worksheets("hidden").Range("e:e").Copy _
       Destination:=ActiveSheet.Range("e1")    
   
End Sub

But you can't let the users insert/delete rows.  And if you add/delete rows or
make other changes, you'll have to remember to update that hidden worksheet--if
you don't update it, you could be in for a world of work to fix all the
formulas/values.