Mulch decomposes. This sounds like find material to use as mulch in your island bed. Soon it will be decomposed enough that you need new mulch. Turning or mixing it should not be necessary. Worms, microbes, and rain will transport the nutrients from the decomposing mulch into the soil where it can be used by the plants. All you need to do is add more mulch when necessary.

purpleinopp: Thanks for your reply! So since it is already decomposing, and I have soil mixed in, is it ready to plant in now? I assumed I had to prepare the "soil" by amending it with other organic matter to balance out the nitrogen, acidity levels? Figured it needed a couple seasons to be ready! If I'm wrong... would love to hear it!

If I am not mistaken the decomposing woody material ties up the nitrogen until it is fully composted?
Maybe add more nitrogen?
More investigation is in order, LOL

Mulch is decomposing wood. I would start planting ASAP.

Thanks all! I planted tomatoes and peppers...we'll see what happens! ( I'll do flowers next year if it works out.) :)