accept_message() is called before any signature was checked.

Serious gas efficiency issues.

Nice-looking separation by functions, but it could be very gas inefficient and not appropriate for the smart contract code.

Multiple calls to get/set storage waste tons of gas.
e.g. setset_storage_seqno(stored_seqno + 1); set_storage(1, transactions); set_storage(2, confirmations); etc.

is_confirmed function is O(k), but could be O(1)

_ add_owner(owner) iterates through all "owners", just to calculate the number of owners

and so on

It is impossible to create a wallet with more than two owners.
There are methods like `add_owner` but they are never called.
Signatures are not checked at all.
I conclude the given problem is not solved.