Base (plinth) for rotted post?

How old was it?

That's a career in a wet place. You can shorten the post with a block at the bottom if it's all fastened together. A piece with the grain running horizontally will not suck up as much water at the beam top, but the shrinkage across the grain can be appreciable. If the whole thing is clad, do yourself a favor and use something to stop the water from wicking up the post. (gasket/treatment)

Now I am thinking maybe I should use an undersized block to take the weight of the post, but build a melamine form around the area (only about 4in high) and fill it the entire footprint with fiberglass resin. That should keep the endgrain from soaking up any more water.... unless there is an easier way for this?

The whole deck will get two layers of ply, cement board, waterproof membrane, and then tiles. But the underside of the ply (Sturdifloor T&G) will be open to the elements. I won't be able to paint before installation, but should I paint the underside before installation to guard against insects and moisture absorption?

Thanks, Shawn

Use a piece of HDPE cut to the same size as the post. It will never rot and you can use wood working tools to get it to the right size.