That actually is a valid concern. They announced the date of the event and news spreads fast, especially around the time of E3. There will be thousands of gamers watching for footage and I really hope that CD Projekt has accounted for that. I would rather not feel like I am watching a Nintendo Direct conference with constant buffering and needing to restart several times. Also, you might want to watch the previous time there was a CD Projekt conference, they tend to be pretty meaty for a appetizer.It's not so much on whether the content will live up to the hype, the larger problem is that the servers will melt within seconds.
Any reveal of "unseen footage" of Witcher 3 will attract people like girls at a Justin Bieber concert.
It took mere hours after the reveal of this conference and you instantly saw topics from all different sort of gaming sites covering this news and with a week to build hype combined with the fact that E3 is around the corner people (which generate more Witcher hype) people will loose their shit.
Combine all that with anything related to live coverage i.e. internet and you'll have a soup of disaster. In terms of the actual conference I expect it to be solid overall though don't expect too much as the main treat will be featured at E3. This will more serve as a appetizer to the people attending E3 and serve as a bit of a consolation price for all people who can't attend E3. Again, I'm fine with all this but I bet 50 "Marcin" that the server won't last long.
CE details and pre-order info is honestly quite moot, people ultimately want a good product and game play is a proof of that rather than arbitrary goodies, though it's nice to know we will probably get a CE that actually also include the game (I've heard that is apparently a rare thing these days).
Regarding the CE, yeah the game is the most important thing but extra goodies are always welcome. It is the last game of the trilogy after all.
Also, give CD Projekt a bit more credit than that, they have proven themselves to care about their fans (though they have been stingy when it comes to footage), so the CE will contain the game too.