Hello Matthias,
1. You cannot prevent participants from leaving fullscreen
2. You cannot disable the zoom function of the browser
Points 1 and 2 fall under the more general idea that scripts cannot take control away from clients (same for, eg., forcibly placing the cursor at a specific position on the page)
3. You can use .css("font", "italic small-caps bold 12px/30px Georgia, serif") on specific elements, or add CSS rules to files in your project’s Aesthetics folder. Note that those are technically preferences: if the font is not found on the client’s system, or if the client explicitly set their browser to overwrite sites’ preferences, the browser will choose a different font (this doesn’t happen very often when you specify a standard font)
4. I’m not sure about this one, but it sounds like it would be beyond the scope of standardized browser specifications. Each browser and/or operating system probably has settings to enable that behavior though, I guess you could invite your participants to follow instructions to manually disable notifications while in fullscreen
Jeremy