"WHEC (later WAXC) had an FM CP that was never built."
The Broadcasting Yearbook says WHEC's proposed FM, which got the callsign of WHEF, may actually have gotten on air briefly in the late 40s, with 65 kW on 96.5 (later WCMF's frequency). However, that may or may not have been an accurate listing--Broadcasting was known on occasion to treat stations that their owners told them WOULD be on the air by yearbook publication time as having signed on, and it didn't always come to pass--and they didn't always check until years later. Warren Doremus, who was around and programming WHEC back then before moving over to the TV side, and is still alive and well and living in the area, would know for sure. If WHEF ever signed on it probably lasted only a short time. A lot of successful AM stations threw their FMs away in the late 40s and early 50s...case in point, WAGE in Syracuse had an FM sister station on (I believe) 96.3 with about a 5 KW ERP signal from a downtown antenna site. It signed on with afternoon and evening simulcasts of the AM station in about 1947, but it was long gone by the time Meredith bought WAGE and changed it over to WHEN (AM) as sister station to WHEN-TV in 1954. WHEN CE Bob Ardner, who babysat the FM operation while it was on the air, showed me an old log book for it, and told me he tried to talk former owner Frank Revoir out of shutting it down and turning in the license (not to mention turning back the CP for WAGE-TV Channel 10 in 1948, which never got built). But Revoir was convinced both FM and TV were fads that couldn't ever turn a profit. Oops....
WENE, of course, finally got its FM sister station years later when Merv Griffin put WMRV on the air.
As to why Chennel 15 was never built, Bill Pearce told me once that apparently WBBF owner Maurice Forman (who had the CP) became convinced UHF would never compete with the two strong VHFs in the Rochester market. Everyone saw what happened to WBUF in Buffalo in the days before the all-channel tuner legislation when even a network's money couldn't make a UHF station pay against a pair of VHFs. Both Forman's Star Broadcasting Co. and RAETA were in the fight for the last VHF in town (Channel 13) for years, along with a bunch of other local firms, before everyone called a truce, the all-channel bill made UHF more viable, and RAETA went off to build Channel 21 while someone (maybe ABC according to some) brokered a deal to get 13 on the air as a joint venture and an ABC affiliate.
WVET also had a CP for Channel 27 in Rochester, as a way out of its sharetime agreement with WHEC over Channel 10. It dumped that CP in 1961 when Veterans' Broadcasting was able to sell its share of Channel 10 to Gannett and buy WROC-TV, then on Channel 5, instead.