The sitting on top of each other wasn't a problem for us last year - once the eggs were hatched they just all went about in a group with a mum and an aunty, all quite happily. All happy hippy sharing and chicks didn't mind either.
Hens can dehydrate if the weather gets really hot and this can be a problem - if they're inside or in semi-shade they should be ok but keep clean water and grain nearby (I always use mixed corn with some black sunflower seeds in which they love) but you don't really want the other hens coming in and eating it all so best to put a free-for-all bowl nearby. I often put a lump of wet bread just in front of them (within pecking distance) each morning too as its easy for them to eat and it makes sure they get liquid. We used to fuss and worry a lot, then we came to a place where we inherited some wild hens that just sat under bushes and in workshop cupboards and had all their chicks - they didn't want attention and they all had their chicks and lived quite happily, teaching their wee ones how to clamber up to roost in the trees when they were about 4 weeks old - it stopped us fussing. They can take care of themselves quite easily.
By the way, ebay eggs - we used to let them sit for 24 hours tried not to move them etc - now we just stick them under the hen/duck straight away - we've done it many times and it didn't make any difference to the number we got - get a good supplier though - a couple of times we have had surprises (not what we ordered).
Also, just for the record, the cooling of the hen in a bucket of water to take her out of her broody state was something we learnt from watching River Cottage - pressumably Hugh got that from chicken whisperer Pammy Riggs (its not something I made up)! Happy chicking!