I really don't think I would have tried this on a two layer job, a solid ground plane is really, really helpful when trying to pass EMC apart from anything else, and it will make high speed things like USB altogether better behaved.
My starting point these days is 4 layers, unless it is something utterly trivial with nothing at all fast edged on it, and moving to 6 does not take much convincing, anything with RF on the board really needs a plane, but see the datasheet for the RF module for details around the aerial.
The premium for 4 layers in reasonable production quantities is very small now, so unless this is incredibly price sensitive (In which case there are cheaper options then FTDI), I would start by burying a ground and 3V3 plane, fixing the pin 26 issue and sorting out the USB routing.
Regards, Dan.