You would always rough turn the form first and then do the microwaving.
You need to understand how wood works: basically you have a bundle of straws. When the wood is green, they are full of water, both as in filled with water as well as they are saturated with water. When you dry out wood, you first lose the water that fills the straws. Once that is gone, you lose the water that saturates the material. During both steps you end up with straws that shrink in diameter, which causes shrinking of the timber. If the timber is still quite solid, then the loss of straw diameter on the outside of the piece isn't so bad, because they can move inwards, and all that happens is you lose some of the diameter. However, at the core of your piece, that doesn't work and the straws start to come apart from each other, hence why cracks generally are radial in nature and start close to the pith.
Now, once you have rough turned your hollow form, all the straws can move. Instead of cracking/checking/splitting, you simply end up with some warping. If you can manage to mount your hollow form right on the pith in both headstock and tailstock, and keep the walls to even thickness (which must include the base!), and the wood was evenly grown (i.e. the pith is roughly in the center), you may get lucky and end up with very little distortion.
I do have to warn you, though: depending on the type of wood you have, this works more or less well. Some time ago I turned 4 end grain bowls from plum, and they all developed huge cracks, in some cases about 1" wide (on a 9" bowl diameter). I didn't mind, because I can always cut them up, put some other wood in the gap, and glue them back together. YMMV.