Can multiple parties authorize single submission via the command service?
This is not possible via the Ledger API. The approach usually taken in this cases is to describe the delegation/interaction in pure DAML and set up a choice that a single party can exercise to trigger a process involving many parties.
This is not possible right now. You need to “collect” the authorizations over the ledger via intermediary contracts that add all the required authorizers as signatories. This process is also not interactive, so you have to prepare the transactions upfront.
Both of these limitations could in principle be lifted in the future, but I don’t expect it to happen very soon.