Double ugh. We're getting tangled up by limitations of MySQL when this was designed (lack of transactions and row-level locking) and limitations of the Everything code (writing the entire node at once). I don't know if it's possible to truly fix this stuff without fundamental changes. Maybe I should just focus on making a shared nodelet cache.
Anyway, as I understand it you want to get the node, keep a local copy, do the updates (which currently all modify the version of the node in the cache), and then compare that to the original. Unlike the current in-memory node cache, a shared one based on Cache::Mmap would not update the node in the cache until it is explicitly saved back to the cache, i.e. in-memory updates do not modify the cache. I'm not sure if that helps any or not. Depends on what the update code does.