Haidia: The Demon Underworld should at least answer a few of those. For more depth...
Portals: Considering the description, probably a hybrid between a Stargate, the Dark Portal, and a Roman archway with some added GrimDark. Artistic license is in play. Transport through it basically sounds like a Stargate that happens to shunt the traveller through Hell (that whole mind numbing pain/mind shuts down during transit/etc thing). While the portals are generally supposed to connect to each other only, I suspect a determined squirrel with sufficient nuts could pick a different exit point :p
Currently: The Portals are a bit of a forgotten lore piece, I'd think. They may or may not play a role later when one of the Big Bads (or even Big Goods) figures out they can use the Portals to travel all over the world with ease and relative safety. I'd say the one in Salvar is currently under loosening guard considering the political situation there; same for the one in Corone. Status for the one in Raiaera is anyone's guess. There's also always the possibility that other gates lie undiscovered or newly created and unactivated.
Guidance/Activation: Judging by the Haidian descriptions, active portals are controlled via a convenient panel located right next to it. The only requirement to use it is probably a decent understanding of Haidian language or Old Diamonic or whatever. Hacking the thing probably requires additional understanding of Haidian numbering and/or decent grasp of coordinates.
Notice: Probably not until the Portal's been activated and something or other is coming out.
...course I could be mistaking the Haidian portals for something else you're asking about... D: