WNNK-FM is in fact licensed that way in the FCC database, therefore they should use the call letters WNNK-FM. If you look at the call letter history of WNNK-FM in the FCC database, the call letters were applied for with the -fm on 1/1/85. (Unless that is the date the database was started.) Then on 1/8/85 they applied for WNNK without the -fm. On 10/31/90 they applied again for the WNNK-FM call letters. WLAN-FM should use the -fm in their ID. It could be that around 1990, the owners of WNNK used the call letters without the -fm for another AM station somewhere else in the country for a period of time, or to throw competition off with a temporary call letter change. It is done all of the time.
WARM, in Scranton can use just that. WARM-FM in this area must use the -fm. Of course both stations used to be owned by Susquehanna. If the owners of WNNK-FM call letters allow it in writing, there could be a WNNK-TV somewhere else in the country under different ownership as well as a WNNK on an AM station somewhere under a different owner.
Then it can even get more messy if there happens to be a ship somewhere traveling the ocean that still uses a call letter similar to a radio station. I know of one call letter that was requested by station owners, only to find a ship was using it. The ship owner relinguished the use that then allowed the radio station to use it.
Regarding the placement, there is nothing that allows :55 to :05 for the ID. The rule is specific to be nearest the top of the hour in a normal programming break. That can come down to the interpretation of the FCC inspector. I know of an FM station in the Providence, RI market that was cited by the FCC for using their real legal ID about :55 before the hour, played a song and then played another ID using the Providence name at the top of the hour.The station was turned in to the FCC by a completing station.