lambda sensors are very common on these. they eat them. the other big common fault is the imrc,(inlet manifold runner control) the p1518.

its the control unit for the dual stage manifold.dont buy a new inlet manifold because its not that. theres plenty on google about them and even fixes for them. if you google where the cable goes to the inlet manifold you can check it works.

when the car starts the cable should pull and hold until around 5500rpm then release. if it doesnt pull on start up then your diagnosis of stuck open is correct. the car will run like this all day long but you will lose torque low down. above 5500rpm the car will run as normal. as for the fuel leak i cant help with that. further investigation is needed.

if your on facebook join the ford focus st170 owners club as you with around 1500 members you will get all the help you need