If the arbitrator goes offline during those 20 days, the funds will not be lost because the arbitrator does not hold a third private key; the BTC remains locked in the buyer’s and seller’s 2-of-2 multisig.
Arbitrators are required to lock up a security deposit, which can be confiscated in cases of serious misconduct or failure to perform their duties.
So the process may take longer than 20 days, potentially until the arbitration issue is resolved or another appropriate mechanism takes over.
