I'm not sure that anyone can really answer the question, OP. Not wanting to sound rude, but if Apple Support say the diagnostic passed and if the guidance says it will not detect all falls, then I think that's probably the best you're going to get. I mean, what else can they tell you? It sounds like what you are asking them to do is predict the future and assure you that the Watch won't miss another detection. Well, it might, because there is always the potential for it to not detect a fall. It even says that. If you want something to detect every fall, 100% of the time, without fail, without risk that it won't, then to be frank, the Apple Watch is the wrong product for you. Is it unfortunate that it doesn't detect every fall, 100%? Sure. Does it mean that the Watch is faulty? No.
Sorry if this sounds harsh. I'm just not sure what response you're expecting.