It's the weekend right now so I think you're putting an unrealistic expectation on Scirra when you demand a reply, but luckily your questions do have answers:
1) How will plugins be installed since Google took away plugin control?
C3 plugins are not Google Chrome plugins.
2) Will C3 have exporters ?
3) If C3 does not have exporters will the old exporters work?
4) If Chrome breaks C3 will we be able to run it without Chrome?
It's highly unlikely Chrome will implement a breaking change into the browser, whilst C3 is unique the technologies it uses are fundamental and would be respected during browser updates. Regardless, the stable version of Chrome runs several versions behind Canary for this very reason, and Scirra would have a heads up thanks to this.
5) Will game performance be faster on all devices or the same ?
It's been known for a while that the runtime is the same as C2s, so game performance will at least be the same. Due to the constantly improving capabilities of device hardware, you could easily argue that the average performance will improve.
6) Are there any new features besides running on all devices or is that it?
Scirra have stated several times that there will be regular announcements from here till beta - there's more to come.
7) Is this just an editor update or will you have ways to make games even easier?
C3 was originally posited as an editor update, however the most recent blog post hinted at new export options. When you consider the increased multi-platform support (Mac and Linux) and game-dev-on-the-move thanks to mobile support, making games has clearly got easier.
8) Will you finally make a publishing channel to help with marketing games or no?
Scirra readily promote games made in C2/3, just use any of their social media hashtags or reach out to their social media manager Laura.