If your console is on firmware 7.55 or lower yes. If your console is on 8.00 or higher the answer is no.
Since you need to latest firmware version to play online you won't be able to play online with a jailbroken console and there is no way to bypass it like it was the case on the PS3.
Nobody know except the one that will release a new jailbreak, it's as easy as that.
No this is not possible at the moment. For exemple if you want to play Call of Duty : Cold War (Firmware 8.00) on a 7.55 console you won't be able to launch the game.
For the moment there is one method to downgrade but only 1% of us may be able to do it since it require a lots of skills, knowledge and stuff. So no, if you are an end-user there is no way to downgrade your console.
At the moment there is no way to enable the jailbreak and make it stick even if you turn off the console so yes you need to activate it at every boot.
A kernel panic is normal, it may happen 3 times in a row sometime. All you need to do is to wait 30 seconds before power on your console again and it will be fine.