Subtle 2 makes the power undetectable. No one will be able to know that it's her Immortality power that's bringing her back to life.
Insidious makes the RESULT harder to discern, rather than the effect. So when she does come back, anyone seeing her "get killed" would assume that she wasn't actually killed--it only looked that way, or even if she was hooked up to an EEG/EKG, they'd probably just assume it was a machine glitch. It also helps that she comes back to life after three seconds.
Insidious is a tricky little Advantage for attack powers, too. An Insidious Weaken Strength attack would drain someone's Strength and they wouldn't even know it until they went to actually use it! If the power was also Subtle 2, they wouldn't even know they'd been attacked (or if Subtle 1, they'd have had to make a DC 20 Perception check). Mind you, most GM's wouldn't allow this for a PC; it's better suited to actions by the bad guys (IMO).