Those are all good questions! Let me see.
- Look through the itch.io FAQ first, please. Ask if an information is missing, unclear or hard to find. Thanks.
- Yes, SugarCube works remarkably well even on low-res smartphone screens (and CPUs).
- Well, itch.io has been around for almost eight years now. It's popular and beloved. Our admins are easy to reach, and still working on the site. I'm confident.
- You'll have to ask the maintainer of SugarCube, but its last release was less than a month ago. And Twine as a whole is supported by the Interactive Fiction Technology Foundation. More specifically, SugarCube allows players to back up and restore saves to/from files in addition to the browser's local storage.
- Many games on itch.io can be played online. In fact we have explicit support for them. A game exported from Twine can even be uploaded as-is, no extra steps.
- See the FAQ, but basically you just have to click New Project on your dashboard and fill in the fields. Just try it!
You're welcome. Hope this helps. And please look through the SugarCube documentation for added features such as forms and multimedia. Sending e-mail is another story however: itch.io isn't made for e-learning, and we have limits on members sending e-mail through the site.
Edit: I just found your post from four months ago that didn't get any answers. Sorry about that, but you posted it in the wrong category, and anyone who could have answered probably never noticed it. I moved it now, but it's probably too late.