By dehydration you might concentrate it; you need a wet way of cooking, steaming or boiling.

I don’t think so, because spinach shrinks so much with heat that microwaving the spinach you might actually be eating more oxalates in the end. Heat can destroy oxalates though, but I don’t know how effectively. If you want to reduce oxalates, I would suggest eating the foods very high in the substance in moderation. I’m no expert though. I would research it.

In a microwave? No. About the only way(s) to reduce the oxalic acid in things that have high oxalate content is to either use an enzyme (which I have no experience with) or to boil them. If you boil them you can wash away a lot of the oxalates and can add a dash of calcium carbonate to help further neutralize the acids. Why are you trying to dehydrate the spinach?