Regarding the fact that Liara allowed Feron to be captured, I don't think she really had a choice. During the confrontation with Harbinger and Tazzik on Alingon, Liara was forced to leave Feron behind, and escape with Shepard's body. She'd managed to fight off a Harbinger-possessed Collector, and had boarded Tazzik's ship, which was obviously carrying Shepard's body. However, Harbinger had not been incapacitated, and was still fully conscious and mobile. Additionally, Feron had been pinned down by Tazzik, and was unable to fight back. With Harbinger about to board Tazzik's ship, and, by extension, acquire Shepard's corpse, Liara had little choice but to leave ASAP. Had she attempted to help Feron, Harbinger could have taken Tazzik's ship, and both Liara and Feron could have been captured by Tazzik and the Shadow Broker. It was an incredibly difficult situation, and the mission could have ended in failure. Liara was forced to make a decision instantaneously and, realistically, had little other choice.
This is what results in Liara's guilt, and desire to free Feron from the custody of the Shadow Broker. He threw himself onto the sword in order to save both her and Shepard, and she feels responsible for his suffering as a result. It becomes her personal mission and obligation to ensure his safety, and to repay the debt that she feels she owes him.
Personally, if a man were to sacrifice himself in order to save my life, I'd feel incredibly guilty. I'd experience a tremendous amount of grief and responsibility for his suffering, knowing that he'd subjected himself to this in order to ensure my safety. Considering Liara's compassionate nature, I think this is perfectly in-character for her.