Private detective services in 498a cases

Evidence gathered by a private investigator is completely legal and usually admissible in court as long as it has been gathered in a way that doesn’t break the law. That means that any conversations that the PI overhears which take place in public places, in a normal tone of voice, or any pictures that the PI takes of individuals in a public place are usually perfectly legal and admissible.

Yes, it can be admissible and the detective agency may have to come as the witness in the court if required to give their testimony. If the information gathered is in the form of documents from some company (like tel bills, pay slips, IT records etc), then the same can be asked again in CrPC 91 when your turn comes for the evidence

Using the evidences offered by Private Detective Agencies is a grey area. Hiring a private detective is not illegal.