Actually I am also interested in what you mean by "on screens it socks". Do you mean on windows or Mac ( since the latter is your dev platform).
As for your last question, if your font comes with proper cmap tables, Microsoft Font Validator can cross-reference that with the OS/2 table and tell you about language codepage coverage - your fonts having glyphs for a particular language and did not declare support for it, or the opposite, declaring support for a language but have missed some glyphs. The current FontVal runs on Intel mac also.
You are aware that up to 8% of Caucasian males and 1% of Caucasian females have some degree of red-green color blindness? It is one of the most widely-known and common X-linked inherited illnesses. "X-linked" as in located on the X chromosome - males has one and female two, so female can be carrier without being affected.
So one always have to be careful about relying on color to convey information - for example, if you use green-on-red for emphasis, it might have the opposite effect: a substantial part of the general population simply see it as redacted.
I am also of the opinion that legibility comes first - ornamental to the extent of sacrificing legibility isn't.
On the role of font editors being the "gate keeper" of font errors - that's a bit like asking the compiler to check your programming code for you, or asking your word processing software to check your grammar.
Some of it is possible - e.g. word processor high-lights possibly spelling mistakes - but it is not possible nor convenient to check grammar on every 'save'. But there is a a place for dedicated and detailed external analysis. That's why you get proof-readers, software that analyses articles for plagiarism, etc.
FWIW, the glyf test for the typical CJK font takes about an hour and half. It is certainly quite impractical for any font editors to do that every time per load or per save.
Zero-length and not-used FDEFs seems to be surprising common. They do nothing at all and are not needed and can be removed from the fpgm. (Just been looking at a "regression" of FontVal - it turns out that the new FontVal is simply capable of noticing issues the old did not notice).
I suspect some common font editors are just adding template FDEFs, and possibly zero them when doing a final save when they are not used by any glyph. This seems to be the case for CFF and subr's from a discussion a while ago ( http://typedrawers.com/discussion/comment/18288/ )
@Roel Nieskens : not to undermine to usefulness of ttx/fonttools, the subroutinizer seems to be currently CFF only. So it cannot do some of the truetype instruction/contour-level optimizations suggested in this thread.