Sorry, probably your biggest question, about an electrical connector seal, I can't answer it, but for the others, to at least help you along.
If it is the pan, I read that this particular pan does not have a gasket but uses RTV instead. Is this true?
No the pan has a big fitted gasket that goes over the edge a round/rolled lip. If the gasket is the source of the leak, replace it, might as well do a filter/fluid change while you are at it, and get a filter/gasket kit.
Having changed the fluid myself, my guess is the lip of the pan is rolled, i.e. has a rounded edge, NOT a flat flang, thus NOT a lot of sealing area for the RTV. I think you would be far more likely to get leaks using RTV instead of replacing the gasket.
Finally, I know this does not have a dipstick but it does have a tube with a cap on it in the engine bay (says for dealer use). If I want to put some ATF fluid in the tranny, can I use this tube? Also, aside from the dealer where can I get "ATF-4"? Autozone? Pep Boys?
Yes, you add fluid by twisting off the cap on top of that tube and pouring ATF+4 down it using a trans funnel. You also put the special tool dipstick down it to check the fluid level.
This tranny is very sensitive to overfill, that is why it does NOT come with a dipstick and says only the dealer should service it. So I wouldn't add fluid without knowing how much is needed.
It is possible to check the W5A580/NAG1 fluid level yourself, but you need the special tool dipstick (got mine off ebay), a way to measure the fluid temp (the dealer uses the starscan tool to read the temp sensor, I use a trans temp gauge I installed) and the table to show the exact fluid level vs temperature (downloadable off the internet). The dipstick is NOT easy to work with, it is NOT hard at all to dip it incorrectly and get the wrong fluid level reading.
Also, aside from the dealer where can I get "ATF-4"? Autozone? Pep Boys?
ATF+4 is licensed by Chrysler, an oil manufacturer has to pass Chrysler's specs to be allowed to use the name "ATF+4" on the bottle. So as long as it has "ATF+4" as the name on the bottle, you would be fine. Don't confuse that with the small print on the back of the bottle saying the oil companies "claim" it can be used in "ATF+4" applications. If the name of the fluid is NOT "ATF+4" on the bottle, it is NOT ATF+4 and you should NOT use it.
I've seen ATF+4 sold just about everywhere that sells transmission fluid, including walmart that has it as its own generic brand, as well as others.
NOTE: Some of the more specialized synthetic companies (AMSOIL, Redline, Royal Purple, Mobil1) do sell a Synthetic ATF they claim will meet the specs for ATF+4, but NOT exactly thus they can't claim it is ATF+4, and from what I can tell, they do work well. BUT, for warranty purposes, they are NOT ATF+4 and no argument in the world about how the fluid is better quality is NOT going to amount to a hill of beans if a warranty is denied for using the "wrong fluid".