either the plumber has to drill four holes for a radiator or the plasterer has to spend 20 mins mincing about around a load of screws and its not so easy to get a pristine finish :)
Read the forum, theres loads and loads of advice, info on techniques, products to use, controlling suction, which base plasters to use, tips on where to start from, im sure I posted something recently.....
No prep required, just put it on, and feather out around 8 inches or so either side of the join and back from the angle bead, sand down when dried out.
Could be plasterboard over timber, perhaps its got damp and failed, maybe it would benefit from being replaced or at least having some further investigation behind it.
Chipboard wont squeak if you apply D4 adhesive on the joists and glue the tongues on it, its cheaper, quicker and more uniform to carpet over, however it makes subsequent work to get to any pipes or cables in the future compared to floorboards.
It wont make much difference, your looking at loosing an inch and a half with batten and board and you get the benefit of a void to hide cabling and pipework too.
Sadly your plaster wouldnt have done this had you applied it as soon as the render was set, check what you have there is firm or else scrape it off and do as Joe says, PVA the night before then pva before you skim making sure your first coat goes on whilst still tacky.
I find it strange that out of three people adding up, a contractor, a quantity surveyor and yourself that the two people that add up all the time cant and you can!
I think the blue grit really bought me the time and the bonding too, I worked off a hop up too and only use a 14" tyzack trowel to lay on and polish. When I did a really big ceiling that was quite high I used tressles and scaffold boards so i could perhaps 6" depth across the whole width of the...
When I did my lounge ceiling with the same method as you I used blue grit first and had ages and ages to play with the plaster, ceiling was about 3.5 x 3.5, maybe a bit larger, i had a mate help to load my hawk for me and clean up etc.
Removing blown plaster is easy, bolster and hammer, beware though that once you start knocking blown lime mortar off of walls you will be chasing your tail until your orifices are full of dust and you have bare walls. when that happens its time to insulate and plasterboard imo.