It's questionable for Brass to let the retired detective Packey "arrest" a suspect for sentimentality's sake, since Packey no longer is a cop he has no legal authority to make an arrest and it could come back to bite them at trial.
Grissom has no need to obtain the ex-cop's physical fingerprint 10-card for the lab tech to make a manual comparison. As the lab tech later states, because the suspect is an ex-cop their prints were already in the AFIS civil database, they could have simply checked that from the off.
The retired cop Packey shouldn't need to ask Brass specifically to run any unknown prints against the cop husband from the past unsolved murder. A suspect in an unsolved murder should remain in the system until the case is closed even if the suspect was a cop. Additionally their cop would be in the system anyway, as the lab tech later mentions, albeit not as a suspect but for the purposes of eliminating their prints at any crime scene they attend, which also would be routinely checked to eliminate the officers on the new case.