Quick note:
I'm in the middle of extracting sprites at the moment from the 2.3.9 build that was uploaded earlier, and am pondering how I will proceed r.e. updating my
quick reference sheet with the Amazon characters... it takes a while for my aging laptop to extract such things.
As for how exactly to unlock the island, well I went to the villain's hideout and found Grail there, and then talked to her again the next night, but of course I had unlocked Grail previously somehow... the other, much more detailed guide by Archer(?) should be updated soon with the relevant details, my quick reference sheet is more of a 'crib notes' kind of thing that gives very brief summaries of each character's content, not a full list or description.
As of 2.3.9, a number of the Themyscira characters are essentially fillers, with a bit of dialogue assigned to each of them. Only a couple of the characters have any H content, and it's pretty easy to access (just talk to the characters). Note that Pallas' location isn't highlighted, but is in the buildings to the right of the arena at the right edge of the map.
So there's essentially a lot of reading for Themyscira right now, which may or may not be of interest to y'all, and not a lot of H content - no sex that I've found so far, just teasing and groping and a handjob and a titjob. Those wanting to 'get it on' with the Amazons who reside in Themyscira might as well wait for SU 2.4.0.