If you prebook luggage space, East Coast charge £5 per item. Your best bet is to turn up on the day, and ask the platform staff and/or train manager if you can put your big bags in the luggage area.
That's not true. The only charge that exists for luggage is the excess luggage charge - this is charged at the Train Guards discretion.
There has been no change in the loading of luggage into the Guards Vans etc. It is infact encouraged as it makes life within the carriages simpler.
There is also no 'pre-booking' of luggae space onboard, such pre-booking is only required for cycles. The assistance you're talking of is for those who have a genuine need for assistance to/from the train due an impairment which may prevent them getting on/off the train safely or at all. Assitance with luggage because you can't carry it all is not likely to be on offer. Realistically if you can't carry it then no-one else can! Saying that staff will be there to help you at their discretion but they do not have to help you with your luggage if they don't feel confident in handling it due to size/weight.
Simply approach the Guard at Aberdeen who will be located at the Guards Van (Luggage area), ask them if you can put the luggage in there (there is no reason they should refuse) and tell them where you're going. Simple as.
The notice on the door you're talking of is simply to advise people that you cannot gain access to the carriage from that door.
The only reason to refuse storage in these area's is due to restrictions in platform lengths or restrictions in being able to help you at your destination station. The majority of luggage in these area's is generally for a main station such as Waverly, Newcastle, York, Leeds. Smaller stations not so much as the guard will have certain area's of the train to carry out despatch and catering crew can't always be up that end to help.