Heyo. That's certainly the longest intro I've ever seen posted here—I'll do my best to address your points. I'm a completionist gamer at heart, after all.
- It's great that you've been familiar with the SCP wiki for awhile! Just a quick heads up though, the site has changed a lot in the past few years, quality expectations-wise. Remember to pay attention to how old an article is when deciding if you'll use it as a guide for your own writing; you can (and should!) check article age by clicking the "History" link at the bottom of a page, and looking for the page creation date.
- When I talk about the site with my friends, I pitch it as "sci-fi/horror, with some fantasy elements". If you like reading about the alien side of the Foundation, you can take a look at the pages tagged with "extraterrestrial" to see what we've got.
- If you're interested in helping give feedback, definitely familiarize yourself with the resources available to writers in the Guide Hub. Also, it's important to have a good idea of how the community responds as a whole; If you're looking for SCPs that are a good example of current site standards, I recommend reading the high-rated Series II and III articles (SCPs with numbers from 1000 to 2999). There are many inspirational articles in Series I, but articles written six or more years ago are less likely to be up to current community expectations, even if they've got a lot of upvotes!
- Re: Twitter and D&D, we've actually got some people on the site involved in both! The SCP Twitter is linked on the left sidebar; the groups who D&D (Pathfinder, at one point) hang out in their IRC chatrooms.
- So far as being eloquent and writing a lot goes, consider looking into writing tales. SCP articles and clinical tone in itself is expected to be relatively thrifty with language, so tales offer you much more creative freedome style-wise.
- Double-posting won't get you banned unless you repeatedly do it after staff have told you not to, so I think you'll be fine, given the level of conscientiousness you've demonstrated thus far. If you're ever unsure about etiquette, you can always ask: General Advice for the writing process, Proposals and Policy for questions about site policy.
I think that's everything, but since we got all the way here, I'll ask you something my psyc professor asked me before he sang happy birthday to me in front of the entire 350-person lecture: What is your favorite salad dressing?
I hope you enjoy your time here, and welcome to the site!