We have to assume that the 2-way peg is unbreakable. Same as bitcoin blockchain is unbreakable.
Bitcoins blockheaders can be regarded as an example of a dynamic-membership multi-party signature
(or DMMS), which we consider to be of independent interest as a new type of group signature.
spvp is
new type of group signature (it can replace ECDSA M-of-N multi-signature)
spvp is vaporware and will likely be much less secure than MM.