At 5,9 you should be able to ride a 55 even thought it might be a tad big. I am 5,8 and I ride a 54 but often think I could happily go to a 55. I doubt that you will need to swap the fork, the fender mount hole is probably there in the crown as in the picture you posted...it is probably just needing some fenders added. I would keep the carbon fiber fork if possible. It will be more comfortable and lighter.
My buddy loves his San Jose. Single speed is the only limiter but many people love SS bikes. I rode one of my SS for my 30 mile RT commute and liked it quite a bit but I have no big hills. For me, SS just means that you think differently about speed and just tend to enjoy the ride a bit more rather than hammering as fast as you can go (or maybe you still do that but spin way faster
). I also logged my fastest 100 mile ride on a fixed gear so single speeds are only as fast/slow as the gearing that is on them.