Within days of Grayson Manor burning to the ground an impossible amount of police, medical, and legal process occurs: a body is recovered and positively identified through dental records; Amanda's car is discovered to contain DNA of a person the police weren't even looking for; Amanda forges and delivers evidence clearing Mason Treadwell of an infamous terrorist act and he is publicly exonerated within hours; the break-in at Victoria's apartment is investigated and provides enough evidence to have Amanda arrested and charged with Victoria's murder.
Ben's relationship history with Amanda would disqualify him from investigating a fire at her house, let alone any investigating a homicide. (Ben had actually previously castigated Jack for almost making a similar mistake when interviewing a witness.) Amanda can make a very strong claim of police misconduct and have any evidence resulting from the interview (specifically: Victoria's DNA in her car) be declared inadmissible.