Errrrr . . Blimey!
Well gcse . . to answer your questions (or try to) in order . . . IMHO
1. & 2. Truthfully I don't think there is much recognition or consideration of the law of diminishing returns, but the reason for accreditation in the first place is to ensure that products purchased or supplied, are safe to consume and presented to the end consumer in a safe and legal manner. To do that, accreditation has to work to a standard and to ensure that standards are met audits of standards compliance need to be carried out. Products, processes, production units, management, personnel, etc can all change massively during the life time of a contract to supply/sell, often with many small changes that add up to big ones, so going through the accreditation process over and over again does have some value. So does the law of diminishing returns actually apply in these circumstances or are all the "returns" of value, whether they be negative, positive or neutral?
3. No, I don't think certification organisations should be held liable across the board in the event that a company certified by them is found to have produced a product that harmed a consumer post certification. Typically this situation is most likely to have arisen because a certified company has deviated from their accredited processes, practices, procedures etc. In the very rare cases where the situation has arisen and there haven't been any deviations, then I think each case should be viewed on its merits.
4., 5. & 6. Lesser standards aren't usually lesser in terms of assuring consumer safety, they are usually lesser perhaps in terms of overall scope, overall requirements, overall admin and the overall costs to SME's of meeting these standards. You have to bear in mind that many companies that aspire to these standards are small, often family run, businesses in niche markets producing a very limited number of specific products, so, yes, on balance I think it is appropriate for SME's to use these type of standards, otherwise how would they be able to compete with national and international companies who have the resources to achieve what you call "Upper Scale" standards?
Mind you, having said all that, I come back to my original comment that at the end of the day it is for the business to decide what, if any, accreditation standard they choose to use. For the vast majority of food businesses, achieving the minimum standards set by regulatory authorites is more than enough to allow them to sustain their business and customer base.
Regards
Gloria