I'm afraid I don't keep notes other than the change logs I post when a new version comes out. Upgrading will definitely be a harder task if you don't have some kind of record about what code you've changed. You can at least compare Reborn's code with vanilla Essentials v15 to get an idea (if you need a copy of that, let me know). (I recommend upgrading by starting with a clean copy of Essentials and porting code changes from Reborn into it, rather than backporting new code into Reborn.) While you're upgrading (or at any time), if you find anything worth contributing to Essentials itself, I'm always willing to listen.
When Essentials gets put onto a GitThing, all its script sections will be split up into separate .rb files for easier editing. This will also make it easier to spot changes, not to mention having multiple people working on them at once and being able to immediately make small changes rather than saving them all up for a big release. I don't have experience working in this way, but it definitely sounds like a good thing to do and get used to.
I can't say for sure when Essentials v18 will come out, but I'd be surprised if it was more than a couple of weeks from now. That's not too long to wait, is it?
Definitely! It's been on the To Do list for months, but I didn't want to do it in the middle of a version's development (and this latest version has been in development for an awfully long time, which is one of the reasons why the current process is not sustainable). Apparently Pokémon SDK benefits a lot from a more open source approach, so hopefully Essentials will too.