They are fast back style (on the back of the tube not the sides) and will be at least 10mm and max 20mm below the pucker hole. When I get the Miter correct, that is where they will go :-) Is that sufficient distance?

I would prefer to do this joint with silver and am not sure about filleting with silver, hence considering inserts instead of external fillets.

With the inserts, the joint area should be about 182 mm2 for each stay which is equivalent to a 14mm square. I suspect this is as much area as side mounted stays and to me the loading is more favorable too.