What is the best approach to install crown molding on uneven top cabinets?
I think you have it right.
A mounting block installed at the top of the cabinet face is the way to go. In your previous kitchen the crown molding was top mount. I've often seen that installed with a mounting block or nailer fastened to the back of the cabinet face and the molding nailed or glued to the front of the nailer.
With the issues you described the surface mount crown molding the way you drew it up is the best solution. For extra stability on the nailer you might want to consider building a simple L (actually, an upside down L) mount. Depending on the depth of the offset at the top of the cabinet behind the cabinet face you could fasten a 1x2 so it extends up vertically over the top of the cabinet and then add your nailer so it butts up to it.
Small differences in the height of the cabinets won't matter much and will be covered by the molding. However, you can always use a sander to quickly remove any major differences in height.