I don't think they're going to hide the Mortarch this time, since they already revealed him (assuming that the guy at the end of Part III was actually him).
I don't know how often they do releases for both systems at the same time, but a CoS/Warclans double release might be able to happen at the same time as the final (for now) Primaris wave.
That'd be at the start of September, Beastgrave could be towards the middle after that, then Bonetithe takes up the first bit of October.
Mawtribes will likely be another small release, coinciding with a possible vs box at the end of the Bonetithe releases (solely based on the dialogue from the Tyrant teaser) similar to what happened with GG and Sylvaneth, just on a shorter time scale. Otherwise they're just quick, one week drop.
Warcry gets their new warbands a bit after that (I'm assuming the starter bands are going to stay exclusive for a while yet), then SoB and the Psychic Awakening release in November, with the final Chaos book sneaking its way in somewhere.
I don't think that Underworlds will get anything new this year except Beastgrave, stuff like the ogre release might not even happen this year (we were only promised "at least" one book per alliance by the end of 2019, and Warclans would fill the Destruction quota), as you said, this one's gonna be packed.
Maybe Ogres and the last Chaos book are a dual release, IDK. I'm really not sure how GW would be able to fit all this into 3 months, IIRC, they don't really do any big releases in December.