Easy way to use DE powder is put it in washing up bottle or something similar...and squirt the powder on the tree trunks...but you do have to reapply the stuff if the rain will wash it down.
If all your trees are younsters..250-500g would keep you going for while...it doesn't go off so if you buy larger amount is does get cheaper. I buy it to use for my chickens and in their housing and feed, usually I buy it couple of kg at the time from ebay.
DE has so many uses, so do a little reading and see if it would come handy to buy in bulk.
You can leave the apple in tree to allow them ripen...the damage is already done and the moths larvae has gone...what you see is their exit hole...so there is no more damage to come until the fruit start rotting off. But yes, if you have use for them and room in freezer...why not tackle them now. I don't tend to do much chutney so mine end up making apple sauce..or a pie...or cake...or left to the birds/chickens