The restriction on splitting multi-packs is purely one which the manufacturer tries to enforce, with no legal basis whatsoever. The only penalty for doing so could be a refusal to supply further multi-packs to the trader but, given that there's usually a middleman (i.e. a wholesaler) in between the manufacturer and the retailer, that's wholly impractical.
The only time that splitting multi-packs might be of interest to trading standards is if there's a statutory obligation to display the ingredients of a product and that information is only provided on the outer packaging. (i.e, the trader would then be selling the single items without the necessary information about ingredients). Even then all that the trader would have to do is to attach stickers to the individual items, declaring their ingredients.
The dates on things like onions will be 'best before' dates, rather than 'use by' ones. It's entirely legal to sell products after their 'best before' date. (Indeed, there are some shops and market stalls which sell nothing but over-date products). It's only 'use by' dates which must not be exceeded.
You employer is acting entirely within the law and is to be comended for his enterprise.