I think it's worse than you are making it sound. By your own admission you have a roleplay centric group, and yet you had to condense the 20 days to 12. And even then, I think asking even a roleplaying group to spend 6 sessions essentially near-powerless victims of tedious lashings will stretch their patience to the limits. 10 sessions definitely would.
As for the swarms, lets be clear here. Swarms have always been one of the more broken parts of Pathfinder/3.5 rules, and placing these in a scenario where there is no clear way of combatting them is beyond 'deadly', its malpractice by the adventure designer. As mentioned, without a very specific type of PC in the party, this is a near certain TPK, especially in the scenario where the party is attacked by the ankheg at the same time as the swarm.
Finally, the overuse of monsters with improved grab and high-damage output means that you are likely taking multiple PCs out of each fight, not only threatening a lot of deaths but also being constantly frustrating.
Not even mentioned is that the group supposedly only has 48 hours to get through the island the only fresh water source is miles from the coast.
A more creative selection of opponents on the Island (no flying creatures? how about some goblin primatives?) would have presented challenges to different character types without making it a cakewalk.
My party is a veteran group of 5 members and is on 25 point buy and if I hadn't removed the swarms and fudged in a few cases they would have been wiped out. I question if this was ever successfully play tested without many player deaths as written now.