Question: Can I use my 105 triple front shifter on a double setup? I’d be getting a new double front mech, but keeping my old shifters and putting them on another frame.

I seem to remember that it’s possible, but I can’t remember the details.

yes you can

How? Just leave as is and beacause the mech won’t physically move any further and remember not to shift on to what was big ring? Do the new rings correspond to granny and middle on the triple?

I set mine so that the big ring is at one extreme of the throw and let the “inner end” lie unused. As long as you set the stops correctly the worst you’ll get is a slack cable if you overshift downwards

Yes but might not be able to trim.

I’ve set mine as scaredy did and the shifter wouldn’t shift into the reduncant granny position as it required cable tension to activate the ‘clicker’ in the shifter. Wouldn’t make much difference the other way as you’d be pushing against the limit stop.