The true Resource Island is indeed located within Tai Lake, and one must cross its waters to get there. Without the ability to fly or some special means of traversing the water, it's essentially unreachable. However, a portion of the Demon Race with very low Realms still benefit from the area. Of course, the Spirit Fruits they obtain don't come from the Resource Island itself, but from the flatlands at the foot of the Beimu Mountain Range.
Yes, that forest is where many powerful members of the Demon Race reside. In the beginning, no demons lived here, but as more and more arrived, a Settlement gradually formed. Furthermore, because a large number of Spirit Fruits were planted, the environment improved. The concentration of Heaven and Earth Spirit Charm is so high that it could be called a paradise.
