Planning enforcement: permitted development misuse, appeal vs JR, and personal liability

You acted as Lead Designer and CDM Principal Designer on a small residential project comprising 6 dwellings developed by Riverford Developments Ltd. The scheme was procured as a permitted development under Class MA (commercial to residential) of the GPDO 2015, converting a former office building to dwellinghouses (Use Class C3) without a full planning application. Prior approval was obtained from the local planning authority (LPA) before commencement. Mid-build, the client instructed a series of changes: dividing two of the units into smaller habitable rooms; adding shared kitchen facilities on each floor; and removing internal flat front doors in favour of a single corridor access pattern. You queried whether these changes were consistent with PD rights; the client said they were "minor internal layout amendments" and instructed you to proceed. The LPA has now issued an Enforcement Notice under section 172 of the Town and Country Planning Act 1990. The notice alleges that the actual operational use of the converted building is more akin to a Houses in Multiple Occupation (HMO) or a Class C2 residential institution, that the changes you made mid-build took the development outside the scope of the Class MA prior approval, and that the development constitutes a material change of use without planning permission. The notice requires demolition of the alterations and reinstatement of the building to its prior-approval condition within 6 months. The notice also names the developer and yourself. The client has refused to engage and is muttering about appealing. Their solicitor has written to you asking for a copy of your design records and your written advice. The 28-day appeal deadline runs from the date the notice was served.