i wouldn`t be suprised if all the gas has leaked out by now ,as the age of the car the condenser may well have a few leaks 🙁. try and get your garage to check that for you if they haven`t already
B in your photo looks like a sensor, what you get on door, boot etc does it press down if you push it.
doubt you`ll get an cover to fit other wise it would have been on car when you had it i would have thought
when did you last change the fuel filter and glo-plugs.
and before you noticed there was no oil or antifreeze in the car when did you last check it before that ?
no real dangers but as you have found out it`s the wrong size, take it back to where you got it from and see if they will change it.
good place to get batt from next day delivery too. https://batterygroup.co.uk/
just pop your reg in and see if you can get the same amps 75/700
i would have said fuel filter but as you got a petrol focus the filter cannot be changed. try putting some redex into a full tank and see if that helps 😉