Are there any "sandboxes" for GIT/SVN repositories, that people can use to e.g. test out using the .pkgmeta, or embeding libraries, or just for pushing commits?
Or should I just use 1 of my own (SVN) repositories for all my testing, without actually tagging it for release?
I've wanted to commit to other repositories to fix stuff for patch 4.1.0, but I was rather scared I instead would totally destroy it ..
All of the actions you mentioned are extensively documented in various forum threads, CurseForge KB articles, and thousands of websites (for general SVN/Git usage). If you're actually testing your code changes in-game before committing them, then there's not really much you can "break". If you have a specific question about a specific action and really can't find any documentation that explains it well enough for you, post the question, or ask in IRC.
Also, for .pkgmeta or embedding, just look at any of the thousands of addons that are already doing those things correctly. I doubt there's anything you could possibly want to do in that area that isn't already being done successfully in multiple other addons. Just copy and paste.