Capot: Few reasons; Bleed air required from all three, certain services (hyd/gen) are only attached to particular engines, and jockeying eng 3 & 4 can be used to balance out the thrust from eng 2 so as to present a symmetric load on the undercarriage.
The big yellow things are 'super chocks', also bolted into the ground.
Some of the team from the programme occasionally do a superb talk on all the above and more. You may be able to request the notes from previous talks given at the Royal Aero Society (RAeS).