I don't see any record (at least in contemporary Broadcasting Yearbooks) of WIBG-FM on 97.1. It shows up in the 1947 BY (1946 data) with a construction permit on the old FM band at 46.5 mc. It doesn't appear in the 1948 BY (1947 data) at all, and it reappears in the 1949 BY as being on the air at 94.1, with a sign-on date of February 1948.
As for KYW-FM, it shows in 1947 (1946 data) as operating on 45.7 with a CP for 100.3. By 1948 (1947 data), it's already on 92.5, and by 1949, it's gone.
I'm not sure I'd consider KYW-FM to be the "original incarnation" of WXUR; it operated perhaps very briefly on 100.3 before the FCC reworked pretty much every FM allocation in the country during 1948, but had no other connection at all to WXUR or the later occupants of 100.3.
(Other 1946 FMs in Philadelphia: WCAU-FM 102.7, WFIL-FM 45.3/99.9, WIP-FM 97.5, WPEN-FM 99.5. The FCC initially thought it could do every-400-kc spacing on the upper FM dial, but quickly changed its mind.)