My big attempt at “optimal” validation is now here:
https://github.com/neos/flow-development-collection/pull/335
Also, a simple alternative solution considering Aggregate Validation Boundaries is here:
https://github.com/neos/flow-development-collection/pull/334
Both are worth being discussed, the latter could theoretically land relatively soon, as it is
less breaky and the implementation is quite simple, but adds a dependency on doctrine proxy.