I haven't seen an answer to your question about default left-click actions, so
here it goes.

Open "My Computer"
Select Tools->Folder Options
Click the "File Types" tab.
Locate the XLT extension (if you type "X", it'll take you there quicker)
Click the 'Advanced' button.
Select the "Open" action.
Select the "Set Default" button
Select OK/Close as often as necessary to get out.

this will change the default action all of XLT files

As an alternative, you can use a Visual Basic Script to launch the file instead.
The VBScript can do the checking, and to ensure that you only launch the file
from the script, I'll have to look at an example I did several years ago, but I
believe
you can pass a command-line option to the XLT that you can test for.