After a longer hiatus, I am returning back to the game and would like to know, if I get access to the Greymoor chapter and the antiquities system if I only buy the upcoming Blackwood chapter. I still have ESO+ so I got DLCs covered.
Thanks.
If you have ESO+ Greymoor will probably be available.
This should be true. When next chapter launches, previous chapter becomes a dlc and as such gets covered with ESO+. You won't 'own' it per se and will lose access to it if you ESO+ expires, but should keep access for the duration of it, just like with the other dlc's.
That is of course unless ZOS change their financial model this time, but I don't see why they would.