Yes, every piece is learnable. Some just take more time that others. No one ever composes a piece not to be played; everything written has to be playable by someone even if it is only the composer!!
Most of the bigger parts in the Widor are just chords which can easily be played. The quavers/semiquavers that occur during the piece can also be played with lots of practice to get them fluent and to avoid slip ups with the scalic passages.
Im terms of diffuculty I would say that is difficult to get right (there is more to playing than just the notes) and you would need to spend quite a bit of time on the end passages, however the notes themselves and the key signature pose no difficulty.
Comparing it to FRCO standards doesn't really work. Yes, the pieces in the FRCO list are challenging but the repertoire lists, if I am correct, are not chosen due to difficulty (there are piece on the ARCO list that are easier than grade 8).
Most of the time, it all bottles down to prior experience of playing works like this; I would not attempt it having not played any French Symphonic works before. It also all comes down to how much work you put in...