It may be that comet tie the hands of the managers. I don't know laptops so I can't comment on how often this problem happens but in my general experience for every genuine fault there are three faults caused my the customer. Without that initial defiance I can imagine they would be out of pocket very quickly. That said they need to have a better procedure.
If one of the keys of the keyboard fall off, on a slimline keyboard, then generally it's fallen off due to the keyboard being banged, and that is where the staff might have had an issue, fair use/wear and tear. As on these type of keyboards, it's not an issue of pushing it back on or gluing it, a new keyboard needs to be fitted. If this was caused in transit to the customer, or by the customer would be hard to prove. But if the customer banged the computer, then the warranty would be invalid.
Regarding the vacuum cleaner - I know people that have worked at Argus, and the gripe about returns is that many orders include associated free items/vouchers, that aren't returned. Buy xyz item, get a £20 voucher, person then returns item, but 'says' they have spent vouchers=stealing.
Buy item, get a free accessory, return main item, keep free item, stealing, unless as the OP states, wanted a like for like swap. But the till system is inadequate, so you get the free item again. That's what Argos staff say.
Funnily enough, I was looking through the catalogue, and came across a pond pump that I require, they had two identical, but one with more power, priced at a ludicrous £99! The dearer one was listed as 'home delivery only', where the cheaper one didn't mention that...WHY? Same size, same item, different motor inside, no heavier, not bulky?
