weve just done 18 of them 400mm square 900mm deep. used a hired hole borer to break the worst down and then the double spades to clear the spoil. very easy, more than enough room to get it out
gravel wont bind when you compact it. youd be better leaving it there and giving it a covering of mot. The cobbles can be laid on a sharp sand bed, but i would rather bed them in a semi dry mortar mix
we always wrap the frame in the membrane rather than putting it on the floor. stops stuff dropping through and weeds such as bindweed etc, finding their way up through
i assume your laying on a sharp sand screed. if thats the case no problem. it will compact down a little better anyway. you just wont be able to do the kiln dried sand at the end
in this hot weather the ground and the salbs will pull the moisture out very quickly, so if its not a wet mix they can get loose. as well as the advice above i always give them a little wiggle at the end just to finish off!
you want a minimum thickness of 150mm mot for a driveway, well compacted. you also need to allow for that face that it will be compacted in your calculation. 50mm for the gravel is about right.