Stormblood conditions?

[Niwashi]

... whether you have to have Heavenwards and whether you have to play through Heavensward ...

As far as playing through it, Legion and Enkidoh already answered that. Yes, you do, for everything except the new jobs.

But as for needing to have it, purchase of Stormblood includes access to Heavensward, so HW doesn't need to be purchased separately unless you want to play through it before Stormblood's release. If you buy Stormblood, then once it's registered, you'll have both expansions. (That isn't a pre-order bonus, though. It comes only when SB is actually registered to your account, which you can't do until the 20th when it's officially released. During early access, you still wouldn't have access to HW unless it had been purchased separately.)


EDIT:
correction: pre-ordering SB is going to give access to both HW and SB during early access after all.