If hinting is the only issue that prompts the workarounds, then you might consider installing ttfautohint and just batch-hinting the first-generation TTF font files directly. (In which case, I should think that you can skip the step 3 decomposition.)
Is it useful to group letters across writing systems?
From a design perspective, it can be useful, in order to simplify the synchronization of values.
But from a production perspective, it can be valuable to have these split, for various reasons — e.g., to prevent enumeration of exceptions across writing systems, which just adds bloat (as Andreas alludes to).
Different designers undoubtedly have different strategies for managing these conflicting paradigms.