> Aldehydes covalently bond and crosslink the proteins and irreversibly kill all of the fixed cells.

That is the textbook answer, however these bonds are only "irreversible" as a matter of biochemistry. You can actually break any chemical bond by increasing the temperature enough. The problem for our purposes is that this means destroying the structure.

> There is zero hope that this provides a solution to cryopreservation except in the slice it up and look at it under the microscope sense.

The trick to reversing the bond without damaging the structure would be in delivering high enough amounts of energy with high enough precision to have only the intended effects. This may or may not be physically possible. However, to rule out the possibility completely, we would need to consider a wide variety of physical interactions that are well outside the range of biology and wet-solvent chemistry, in addition to the full spectra of potential biomimetic and biological approaches.

Good luck. When you make an imine with an aldehyde the principal mechanism of irreversability is that a stray reductant (available in spades in biochemical settings) takes the imine down to a primary amine. (This is not found in most textbooks, you just have to know that). Now, a primary amine is a relatively stable covalent bond, and getting selective deamination is going to be especially difficult since there's plenty of amines around. Not the least of which is the other side of the lysine that the aldehyde attached to in the first place.

True! But slicing it up and looking at it under microscopes is, in fact, the most likely means of restoring cryopreserved brains, either as computer simulations or as nanobot-constructed replicas. Doing it this way is easier because such techniques could handle cases where molecules are damaged in ways that make them not function but which make it obvious (from a good microscope reading) what they were before.