> - Please allow M=1. From a usability point of view it makes sense to allow
> the user to select 1 share if that is what he wants.

How does that make sense? Decomposing a key/seed into 1 share is functionally equivalent to dispensing with the secret sharing scheme entirely.

I agree that it may look silly to have just one-of-one share from a technical point of view, but from an end-user point of view there could be reasons for just having one piece of paper to manage. If M can be 1 then the software/hardware doesn't have to support multiple formats, import/export paths + UI (one for SIPA keys in one share, one for HD seeds in one share, one for SIPA keys + HD seeds in multiple shares).

Less complexity & more freedom of choice.