i wouldn't remove diamond storm on diancie or else this team gets bodied by talonflame. i would increase the speed evs to max though, as you don't want to be forfeiting momentum to keldeo and your match-up with gengar is terrible so speed-tying it to hammer it with diamond storm would be appreciated.
not sure i'm feeling the stall weak as you really have a lot of weapons to break down defensive cores, though you are pretty megasaur (and to a lesser degree, amoonguss) weak, especially if it's paired with heatran. i think running the physically offensive spread on diancie might help with this problem, as a max atk diamond storm levels amoonguss (45%-53%) and offensive megasaur (ditto) while doing 33%-39% to the bold defensive megasaur, which puts it in 2hko range with a scald burn and stealth rock. it also helps with tentacruel, who is actually a somewhat annoying match-up for you, and ditto for cm clefable. that would also free up cm for protect or rp. celebi is tough to crack too so you'll have to play the hp bug perfectly and reveal it at the right time or try to nock it off with thund so diancie can get it into 2hko range easily.
another option would be to just run cm psyshock on the diancie and toss a landorus-t over gliscor, as it can kill sd talon with stone edge while the ones that are flying mono attackers can be handled by thundurus. i like this option less as i think calm mind diancie is a less consistent set than the all-out attacker, though i would consider landorus-t regardless.
run hydro pump over icy wind or hp bug on the keld, probably over hp bug actually since you really don't want to cede games to mega altaria. this will also help with your stall match-up and mega venusaur in particular once you burn it. also, don't forget the 0 atk iv.
some commentary on the team-building decisions would be nice.
thanks for the advice anti! I'll update this later with some changes and some reasoning behind choosing each Poke. This was just something I typed up really fast because I was out of time/too tired to go into detail. I did decide to change out Glisc for Lando though as I felt it was just overall better, the intimidate switch in has shown to help me a lot in the past.