Sandalphon does not show up in this particular game.
If you mean hunters, there are government sponsored units responsible for keeping the supernatural in check. Gabriel does have a run-in with one.
I do actually listen to the Transformers soundtrack while writing sometimes.
I'll go by archangel (By the way, the tumblr has a lot of this info if you're curious). Age order, from oldest to youngest is as follows: Raphael, Israfel, Auriel, Michael, Gabriel, Sarethiel, and Hanael.
Israfel adores Gabriel.
Michael used to openly adore Gabriel when he was young. Now events have changed his attitude and he more often than not ends up in arguments with Gabriel.
Sarethiel is a riddle, wrapped in a mystery, inside an enigma. No one knows their true opinion of anyone else, but they seem to regard everyone as inferior in knowledge and woefully blunt and clumsy instruments with no keen sense of perception nor style.
Auriel tries to keep her opinions of others close to herself. She's fairly neutral on Gabriel, preferring to adhere to a 'leave and let be' approach.
Hanael likes to think positively of everyone (who is an angel). Gabriel she respects but also sees as wrong because they often oppose Michael.
Raphael doesn’t like to think of anyone in terms of emotions, rather as tools that each have their own unique use. So Gabriel is a sometimes useful tool to him, sometimes not. They are the oldest archangel currently (Ramiel was older).
Alice is one of the potential partners for Gabriel as a cop. She is also a RO. There's info scattered in the thread and throughout the tumblr, or feel free to ask specific questions if you want to know more.
A couple people have been asking about offspring for Gabriel (with Michael and Ramiel) specifically. At this point, it's not going to be included in the story, or at least the offspring themselves. Discussions about children can occur with every RO. However, most of the ROs aren't available until Daniel turns 18, at which point there are bigger concerns--this is the point at which the prologue begins.
As for your second question, Yes, it is possible to kill Lucifer off. Most of the characters can die or be killed off.