You've definitely hit on some of the core issues: who determines assessments, source of program accreditation, employer buy-in, etc. While the Mozilla/P2PU approach addresses the first point in part and the last point in full, it actually seeks to circumvent the the "accredited by something higher" issue and I think this is probably easier said than done.
I'll note that one of the reasons +Rovy Branon
and I have started using the terms micro-certifications and micro-credentialing are to highlight the fact that badges (with all the baggage they carry) aren't the only way to approach the grain size issue that our current modes of certification present. This is a distinct but closely related issue to the bigger framing problem of providing a common signifier for skilled/employable but un-credentialed people to utilize with prospective employers.
A third related issue involves generally qualified individuals who have moderate continuing education needs (i.e. acquiring new skills or updating existing ones, preparing for a horizontal career change, keeping current with the cumulative impact of progressive changes to a field, etc.). I raise this point just to indicate that we're looking at a tangle of related issues that I believe are tractable, but will likely require a host of interconnected solutions.