The battery inside the key is for the remote keyfob to lock/unlock the doors. You can use a passive key, one without the buttons, to start the car and it has no battery in it. The battery should not have made a difference to starting the car. But at least we have ruled out a dodgy key rfid.
Forscan is a good option, you should be able to get the DTC's read with that - from the
PCM and Instrument Cluster at least. My experience with the WQ Fiesta on Forscan though is that it won't read both CANBus's without a bus switch - the Fiesta has a main HSCAN bus with the
PCM, IC and maybe one or two other modules, and the MSCAN bus has the GEM and the Audio Module and the
ABS on it from memory. (Those Fiesta's had Voice Recognition modules in the UK, and an Auto-Shifting version of the IB5 manual gearbox - an earlier version of a dual-clutch transmission)
If you need to do a full PATS recode of the keys or fobs, Forscan I think can help you with that too. (PATS = Passive Anti Theft System). You mnight need a licenced copy of Forscan for that though, as the free version I suspect will likely have a useful feature like that disabled.
Luke