no reason to feel stupid and it's understandable you'd have a lot of questions at this point.
have you checked the prices on dart frogs yet? i've seen them sold for anywhere from $10 to sky's the limit. i will say that $10 is a lot less likely than $100 each.
definitely stick with one species from one local. these guys are quickly becoming threatened in the wild and it might be sooner than later that hobby specimens might be the only thing left. it would be a shame if none of them were true to wild form at that point from cross breeding. let alone the risk you take when you mix species.
you have a lot of questions your answers will not come overnight. understanding them would be a daunting task even for the most intelligent person so fast.
i would definitely get yourself a fruit fly culture and seed your tank with springtails as soon as the tank is planted/finished and ready to establish itself so you have adequate food and experience breeding food for them when they arrive.
for now i would start out with a small build and see how it goes. it will cost you much less in materials to make your mistakes on. once it's established you can pick up 1 or 2 darts to stock it with and start on another tank while you familiarize yourself with things.
definitely look up quarantine and lamisil treatment... chytrid treatment... as well as fecal testing and such. rich frye has touched on the subjects a bit over on dartfrogz.com. dr. frye (riches brother) could help with fecals also.
do not trust anybody at a local pet store until their info is verified and triple checked. not that every pet store or employee is horrible just that people there are typically low paid and just making do. chances are they don't know half of what they're forced to answer on a daily basis.
not wire mesh... fiberglass replacement screen for windows.
abg soil mix is common for darts. many people utilize bromeliads for them also.
6500k lighting should be good for most plants. special lighting for the frogs should be unnecessary with the right supplements.
try to email viv concepts. most of the time online sellers are more responsive to emails because they can get back to you when they have a chance to.
water leaks would depend on your design. if you design it with a f/b you shouldn't need to worry. if you choose a separate reservoir for your waterfall and it leaks to the soil side that can be an issue. either way darts like it wet so i wouldn't worry so much at this point about that.
not familiar with the activated carbon frogs. activated carbon does help keep water clear of organics and smelling clean but with moving water and dart frogs it's probably not necessary.
you are pretty much correct with the tightbond method.
definitely check out the stickies in all the forums on this site. you'll learn a lot in no time. also utilize the search function to the best of your abilities whenever possible. it's probable you'd find the answers to your questions before anybody could respond. i'll leave you with this link. it should have more info than you'll want to look through but is a great resource imho...
how it's done... favorite links thread
i did merge your threads and i know it makes it hard to look through but it's easier this way in general i think. i've done what i can to answer your questions. hopefully i didn't miss too much and i've helped clear some things up. welcome to the site.