Rumor: After MotMV is released, you won't be able to buy Volos or MToF separately

[Leon]

The new monster book also conflicts heavily with the Monster Manual (see the characterizations of goblins and hobgoblins for example). That's liable to cause confusion. Which version is "authoritative" (as much as such a thing exists)?

WotC products conflict all the time thru all the editions, this is nothing new or unexpected. You even have a Dev masquerading as an advice channel who conflicts regularly with stuff they have supposedly written. Generally one would expect the newer Book to be the one that is "authoritative" but your mileage may vary on that depending the shims of individual DMs/groups.