You are correct. The reason is that you send same jobid and jobheader for both main and devfee connections, the only difference is extranonce
Yes, you are right. Job is global and the same for every miner who works on it within its own nonce space. This guarantees unique mining results for every session. The only event when job id changes is getting new block from the network. Another thing is that server remembers the content of last 6 job packages and corresponding results are accepted as well, though with lower reward.