I don't think x11 needs diff-multiplier settings, as x11 algo didn't have difficulty factor incompatibility issue from the start.
(Only qubit would need that option. Maybe it doesn't need that option at all. You can test it by changing diff-multiplier and see the reject rate from dashboard)
~1% reject is normal. Stale shares (little bit late share) can be rejected, and it can happen to any coin.
The reason why you see ~1% reject only from Dash is because Dash is the most profitable coin lately and pool mostly mine that. Other coins would not have long enough time to be mined.
1. Does diff-multiplier affects whole (X11, Qubit, Quark) algo settings?
2. Is your x11 hashrate reported correctly on pool even you set diff-multiplier? Then Baikal's diff-multiplier option doesn't seem to work with x11.
The diff-multiplier is a single setting which I believe affects all pools (there's no way to set specific pools or algos, so I've no idea, but this is my guess). I know I had the diff-multiplier set a couple days ago, noticed we were mining Dash, and that I was seeing around 20-30% rejected packets, so I shut off the diff multiplier and it fell back to 0-1%, so I thought it was that setting. Now I have it on again, though, and all seems to be working, so I'll keep an eye on things and let you know if there are more issues