You've really hit on the essence of selecting a scope: Buy one that is right for your shooting needs. Now let's see if I can answer some of those questions.
Water resistant is alright but are you really going to expose your scope to huge amounts of water? I've had mine in a shooting box for twenty or more years and it's been rained on plenty, especially at Camp Perry. Never had it fog up, ever! Most scopes have the internals filled with an inert gas like nitrogen that prevents moisture from building up and condensing on the inside.
I've got a Bushnell Sentry II in my box with a 20X50 eyepiece in it. I can easily see .22 cal holes at 25 and fifty yards. In fact I can read the fine print on the bottom of the target from the shooting line. A hundred yards I can still see the holes but that's about the farthest I'd want to use that particular scope. A zoom lens can bring in a distant object but you sacrifice light and clarity. Sometimes that is not what you want to give up especially in low light situations, like many baffled shooting ranges have today. I don't see a need for a zoom lens for pistol shooting. In my Highpower Rifle Kit bag I have a 20X-45X Zoom Lens on my rifle spotting scope but the distances are far greater and light gathering is usually not a problem.
Straight eyepiece or angled is up to you. That really comes down to personal preference and how you are going to mount the scope for shooting. My pistol spotting scope is a straight through version but my scope is mounted to my pistol box lid and when it's open, straight is easier for me to see through. I see lots of folks out on the line with the 45' angled lenses they seem to be alright for them. For rifle shooting the angled lens with a long eye relief is better for me because when I'm all strapped in my sling and have my heavy coat on my scope acessibility is limited and the angled view is easier to see through.
No, I don't think sacrificing power for clarity is a good idea. I can see the holes just fine through a low powered high clarity scope but the same is not true the other way around. It's the light, you need a lot of it through the scope to see clearly and focus properly.
For pistol shooting, I prefer the scope that focuses by rotating the eyepiece. I've had both and the ones with the separate knob always seemed to get screwed up when ever I touched the scope just to adjust the field-of-view image.
A 50mm or 60mm scope will do just fine for the distances you expect to shoot your pistol. The bigger sized lenses are more for longer distances and using the zoom lens, they gather more light into the scope and the bigger lens means less light is lost when tweaking up the zoom.
You probably can find a scope with many of the features your looking for incorporated into it. Do you have anyone at your local range with spotting scopes in their gear? I'm sure they'd let you take a peek through theirs to help you find the brand you want. I've done it a hundred times out at the range when someone asks, "Can you see the holes down there?"
Hopefully I've given you some more information that will help with your quest in getting a spotting scope. It is darn hard to buy one without seeing through it first. I know several top brand names that for one reason or another didn't work well for me.