In answer to your Q's - this particular spot is the only 'ideal' spot I can think of because my planned cache will take some time (up to an hour possibly) to get into, so cachers will need a bit of shelter from the inclement British weather whilst they're solving it. The terrain will also make access pretty tricky making it an all-round high cache, finally, the cache could do to be away from the direct effects of the elements as it's not 100% waterproof (without storing inside another box, which would be fine I suppose). The river flows under a motorway bridge and the huge retaining boulders on the river banks provide some great hiding spots.
I wouldn't be so cheeky as to ask the other CO to archive their cache! He's a big player in this area and has hidden nearly 400 caches for the rest of us to enjoy; this particular one is part of a series and is very popular because it's an easy find at the side of a easy access PF.
I haven't had chance to speak to the Reviewer yet, I'll know more then, but I can't imagine anyone would be daft enough to get the 2 caches muddled up, or try to cross the raging river, or climb across the under side scaffold of the M'way bridge (although we tried that many times as kids!)
Some interesting stuff about saturation on this thread incidentally. As a newbie to laying caches, I've got to say that in a saturated county, it's hard not to tread on peoples' toes. I've got a couple of dozen planned out and I'm wondering how many will be rejected, especially when there may be multi-caches knocking around that I haven't found yet.