It would cost Apple more to perform those tests than it would to replace your watch, so from a financial point of view it makes more sense to them to replace the watch.
Also, do you REALLY think they will have the equipment and certified personell to do that in-store? After 6 months when stores start keeping stock of "white box" watches like they do with phones, they will be replacing on the spot like they do with phones.
It takes too much time and effort to do what you've just said. A lot longer than any "genius" (I hate that term) appointment.
You say the seals will be "well above" the IPX7 rating so if water gets in it shouldn't be replaced under warranty....you do realise that Apple is not perfect and there is such a thing as defective seals/manufacturing defects. There was a thread on here by a guy who had a gap at the top of his screen and from the picture it looked like there was a section along the top that had the seal missing. These kinds of defects will let water in and will be covered under warranty.
Heh, they don't do the analysis in store, obviously. They have an RMA department where these get shipped back to and technicians pull them apart to determine root cause. Now, the cost-of-materials at $84 sport watches, they may just toss, but the watch and editions, they're more likely going to want root cause on.
If the watches are designed well above IPX7 and there's water ingress, you've almost certainly done something over and above what they've warranted. (barring anomalies of component failure - which will get found out when/if investigated).
Me, I'd expect the warranty policies to be similar to those of the phone...but those that want to gamble, sure, go ahead.