I havent got time for a long post this morning,but just thought I would add on the handling front that certainly the YS53/later T53 had pleasant handling characteristics,much better in my view than the Twin Astir we had at one of my clubs.
Personally I do not think the T53 would have been a very good glider to take over the Mk3 role...the K13 would have been ideal,being easy and safe to fly and with good vis from the rear seat and also would have fitted in ok with the standard launching gear then in service.
But I agree the 53 would have been good for the AEF role.
rgds LR