Duke,
It isn't a worry that the shopping area will become busy with people, that's great if it happens. The concern is that if the equation is wrong Marple will become congested with cars which is not great and even may serve to put people off coming into it because of the vehicular traffic.
Well, I think it's safe to assume people will arrive by car so yes there will be more vehicular traffic into the cetral shopping area
To change the layout of the carpark on Derby st, I think there is two choices:
Single level car park – remove the exit onto Hibbert la, landscape the whole area to turn the exit area of Derby st to be incorporated into the car park. This will add around 16 spaces.
Multi storey – remove exit onto Hibbert la, a ramp be built to the 2nd level on the exit area of Derby St, parking under this ramp, this will add around 8 places on ground level & 20 spaces on level 2.
I don't see a small Asda / Waitrose / Aldi creating that much more traffic. The Aldi at Rommily doesn't get as busy as the Co-op car park, the Waitrose at Cheadle is the same, a smaller Asda again would not cause a huge problem, in fact teh one at Hazel Grove does not have a massive car park.
Remember that there are two under utilised car parks, one behind the fire stations & the other oppositet eh hatters.