this will allow you to do a single click selecting only MTEXT on the current Layer:
(setq s(ssget "_+.:E:S" (list(cons 0 "MTEXT")(cons 8 (getvar"clayer")))))
this will do a save:
(command"_.QSAVE")
this will close drawing:
(command"_.CLOSE")
To apply your code to multiple drawings, create a SCRIPT file (name with .SCR file extension) that includes loading of your lisp routine and running the command ie:
(load"MtextUnformat")
MtextUnformat
Then download the free AutoScript app to have this script file applied to multiple drawings:
https://www.cadig.com/products/autocad-script-pro.php
But you'll have to modify your code because it won't pause for you to select your MTEXT.
You can change it so that it just selects ALL MText on the current Layer and then cycle through those MText objects selected (if any) to apply the rest of your code:
(setq s(ssget "_X" (list (cons 0 "MTEXT") (cons 8 (getvar"clayer")))))