Hey RobotCommission!
1. Yes, you are correct, it is one Dangerous Action Roll and damage is taken from rolling under. Exploits and System Responses still apply.
2. The rolls are Dangerous Actions as point 1. Resilience Damage can come from whatever the GM feels best fits the computer system and the narrative. Flatlining is very possible.
3. Yes, the Firewall DS is listed as a target for PCs to beat, if up against an NPC nodejacker. It can also be used as an indicator of how secure/protected the PC’s console is - a DS10 nodejacker NPC without intrusion software to give a bonus will struggle to gain access to a PC’s DS12 console, for example.
I hope this helps? :)