Normally when the starter motor fails its signs that the dual mass flywheel is on its way out as it starts to break up and it jams the starter. I see you say you've since replaced the DMF so that could be why the starter was knackered.

Take it to Ford and get them to plug it into their diagnostic machine, they can do live readings so they test the output of each sensor while the car is running.

An hours labour at Ford will be cheaper than changing bits willy nilly and having a few weeks of head scratching.