Sunday opening - wouldn't this conflict with the FHSoc's monthly market at the station? Also, I'd be a bit concerned that people might hold off coming into town until Sunday, skipping Saturday completely and possibly not helping the situation!
Market clusters - I like the idea of the clusters of types of market, but I'm not sure FH could sustain a weekly non-food market. I used to attend the SEE3 market outside the Pools, which never really took off (I believe a combination of difficulty securing dates from the Pools operator, and limited marketing/awareness).
I would have thought that the only quick solution would be for struggling traders, where applicable, to pitch up themselves. Surely the Horniman/market organisers would prefer to relax some rules regarding products on offer in order to avoid unhappy traders in town? I'm sure that sweets would go down an absolute storm up there, considering the number of families milling about.