They aren't in a relationship, but an optional scene shows Riccardo slapping Daniella repeatedly during a makeshift interrogation; the noise carries into an area Fiona has to visit, though you can avoid the actual scene by not following the noise.
No, but the story involves characters TRYING to sexually assault Fiona. If Fiona dies to the second stalker, Daniella, the audio played seems to imply SA is what happened.
Daniella, the emotionally stunted maid, licks a large shard of glass in one scene. There's no blood, but it's implied she's doing it to feel pain (which she cannot).
Though her sexualization is creepy and she is quite young, the player character Fiona is canonically 18 years old in both the Japanese and English scripts for the game. She is not a minor.
This being a horror game, of course there is. These include a stalker surprising Fiona as loud music kicks in, or random things suddenly sending Fiona into a panic attack, causing her to scream loudly.
The protagonist wakes up in a hospital gown later in the game. There is a cutscene with her on an operating table that can only be seen after your first playthrough.
If Fiona gets hit by an enemy too much or experiences too many stressful things, she will begin to scream and run around on her own as the visuals become difficult to see. This is called "Panic Mode" in game.