Halfway to Heavensward

[Niwashi]

As to being halfway: By simply the number of main scenario quests, it's closer to 2/3 of the way. (There are between 181 to 184 quests to reach Ultimate Weapon depending on which city you started in, and there are another 100 left before the official start of Heavensward, though many of those are in fact already part of the HW storyline.) However, while reaching Ultimate Weapon, you also had to level from 1-50, should have leveled another to 15 for your job unlock and done a bunch of class/job quests, so have to have done a fair amount of other content besides just those MSQ. Though there is certainly lots of content available between that milestone and HW's start, the only part that's required is the 100 remaining MSQ. So I'd consider you at least 3/4 or 4/5 of the way through, probably more.

(Then again, that's compared to reaching Heavensward. Reaching Stormblood would be quite a bit further. While you're quite a bit beyond halfway to HW, you might be around halfway to SB.)

And no, Coil is not required for the MSQ. (Apparently there's one section later in the story where you'll get slightly different dialog depending on whether you've done it or not, but that's it.)