Selecting multiple words via AppleScript in PowerPoint
You are very close. Let's say you want to select words 2 through 4 of your text frame of aShape:

Where you have this:

select word 2 of text range of text frame of aShape


change it to this:

select words 2 thru 4 of text range of text frame of aShape


You could put in any number you want in there, not just 4, of course. If you might need to specify a different ending word every time you run the script you could extend this script by displaying a dialog box and asking for input.