If I’ve understood correctly we’re now faced with a secondary problem – ie that to properly conserve the structure not only does the bank need to be extended but also that the whole thing needs to be taken down and (correctly) reassembled. There are precedents for that sort of remedial work (think the Leaning Tower of Pisa) where unsuitable materials or techniques have been used on works of art etc. Could we ever hope that English Heritage would embark on such a programme at Trethevy...
The alternative is stark though. As you say, the structure will one day come down – if nothing is done now to stabilise it that’s likely to be sooner rather than later :-(