MM left DTHQ a while ago, I believe drum parts were already finished then, but he's just posting about it specifically now (remember he had to drive from Boston to NY for the sessions and isn't/wasn't recording at his home).
JP posted a couple pics with DT15 in the captions, one of which is the one with the 8 string, a few weeks ago, and then Jordan posted that other pic about working on DT15.
My bet? Drums, bass, rhythm guitars and most keyboards are already done, and there's only leads and solos + vocals/lyrics to go.
As for the 8 string being used or not, he has been hyping this for so long now that I think he's definitely using it on at least 1 or 2 songs on the record. Remember JP is also very into the business side of things and nothing sells more new EBMM MAJESTY 8 string guitars than him promoting the crap out of them on studio videos and live on tour. Imagine if they released the new guitar and JP was like "yeah, I had it in the studio with me and it's a wonderful guitar, but I didn't end up using it at all in the album, oh well... guess you'll have to wait for DT16 to hear it then"