I think the banqueting hall should be made in to a restaurant, it wouldn't be too difficult to do, the thing which would prevent it would be due to the building being listed, as they would need planning permission from English Heritage?
Any changes to the towers would need listed building consent (possibly in addition to planning permission) from Historic England
who have taken over some of the role formally undertaken by English Heritage.
Changes that do not adversely affect the more historically important parts of the towers, or the external appearance, should be relatively easy to get passed. They are "only" listed at grade II* (less significant than the Alton Towers Gardens or some postwar buildings such as the Royal Festival Hall and Lloyd's building in London all listed at grade I).
The poor condition of the towers should allow more flexibility as there is less of historic importance. Merlin will have conducted a survey of the towers assessing the importance of each area/feature.