The most important thing is that you've got a clearly defined study plan, and it sounds like you're committed to following it! So that right there gets you a leg up. Now, in terms of the questions:

1) Regarding resources, you're right, the fundamental concepts are pretty much the same. However, the way in which they're presented can be slightly different on each test, so you're better off gaining as much familiarity as possible with GRE question style, since that's the test you plan on taking. Of course what you've learned from GMAT studying is completely transferable, but it's best to focus on the GRE. I'd recommend making use of the seemingly unlimited question banks on this forum https://gre.myprepclub.com/forum/viewforumtags.php -- you can select problems by source material, if you like.

2) I always think it's a good idea to have more than one hardcopy resource on hand--the Official Guide for "official problems" (written by ETS), and a strategy-oriented guide, whether Manhattan, or Kaplan, or any similar resource that you're comfortable with. There aren't all that many practice questions in the GRE general guide anyway, so you'll want to have additional resources available. You can find some tough questions in the Official Guide, but there are certainly more in those other resources.

3) Vocabulary: It's still an important part of the GRE, so yes, I highly recommend consistent vocab practice. And be methodical about it--whichever system you choose, make sure you're going back at the end of every week or so and reviewing your newly-acquired words to make sure you have them down.
I'd also recommend starting the actual GRE Verbal practice now, while you're working on vocab, rather than waiting. That way you'll be learning new words, while applying them in question-specific context.

4) Be STRATEGIC about your studying. In other words, don't just do practice questions/problems, but look for concepts and creative ways to tackle questions more efficiently. Look for clues in the questions/answers, look for ways to simplify, determine whether a question seems easy or difficult (if it's very difficult, may be worth flagging, and returning to later). And at some point, start phasing in short, timed drills.

5) Be consistent about practice tests. The best are, of course, from ETS, but there's a whole range of available tests that can be very helpful for diagnostic info and practice. You don't have to do one a week, but try to space them out somewhat evenly before the test.

6) In terms of signing up for online courses---they can certainly be helpful, but see how things are coming along. As you move ahead with your studying, determine whether you feel that you're making good progress with the self-study, or whether more formalized structure/feedback would be helpful.

Hope this was of some help to you, and best of luck!