Why wouldn't you plan IFR the entire flight?
Because you want to use the privileges of your newly acquired EASA EIR.
Simply because Eurocontrol will not validate an IFR FPL containing TO or LDG at an airport without proper IFR equipment.
Yes it will. I file IFR FPL's to/from EGLD all the time.
Exactly. Otherwise the utility of an EIR would be seriously reduced if it could only be used from VFR only airfields.