From README: "Disclaimer: this is incomplete task". Also, it looks very slow.

accept_message() is called after ALL signatures are checked. It is very likely than it won't be enough gas_credit to handle messages with multiple signatures.

Partially signed orders dict (orders cache) uses in_msg hash as a key, however in_msg also contains seqno, which is incremented each time, so you will never hit a cache (hash will be always different, even if other data is the same).

Most of the key/signature search algorithms are O(N), which will consume tons of gas. Especially find_signature, which calls heavy check_signature on each iteration.
In worst case it could be even O(N^2) (see find_signature usage in the"do-until" loop).

valid_until for partially signed order can be overwritten by next external message

It counts votes twice. Should be ifnot, instead of if