Is a parachain sovereign account required to be the sender of XCM calls or can other accounts send messages? Are there specific XCM instructions that require the use of the sovereign account?
Sovereign Account: An account controlled by a particular Consensus System, within some other Consensus System.
So a parachain sovereign account is in other Consensus System and controlled by the parachain, not in the parachain, you can't use its sovereign account in the parachain to send XCM directly. But you could use it via sudo or governance.
For Example: There are two parachains ParaA and ParaB. ParaA has a sovereign account on ParaB is
0x12340000. If ParaA is the sender chain, you want to use its sovereign account(on ParaB), you could send XCM from ParaA to ParaB via sudo or governance. In this way, you have the authority of the ParaA sovereign account（on ParaB
No specific XCM instructions require a sovereign account, it depends on what you want to do. For example if you want to do something as a parachain owner, such as open or close the parachain HRMP channel, or transfer tokens from sovereign account, then you need the parachain sovereign account, and you could use it by sending XCM to relaychain via sudo or governance. BTW, if you want to know the difference between
Transact, you could check this answer: https://substrate.stackexchange.com/a/5074/1959