EnbyKaiju

From experience musk oxen are one of the most difficult kinds of goat (yes they are classified as goats in the code) to milk. My recommendation (if this is still v1.21) is to put each lactating ox in a very small pen and just hold down right-click until you get the milk out. Unlike sheep all kinds of goats refresh the agitated cycle very quickly, so you should be able to just pen them in a small space, hold right-click, and you'll eventually get a bucket of milk. Just remember to let go when it happens or you'll drink the bucket, haha.
If that doesn't work after a few minutes of trying I'd suggest waiting for a future generation (2-3 should be enough) and give it another go. If that still isn't working it's likely the mod.

They are indeed! If you're doing a snowball earth run or similar they are one of the only domesticatable animals.
They work off the same framework as goats. So milk, meat & hides, plus the horns. And they go through the same domestication process.

So there's no specific way to set daytime/nighttime length, but there are a few commands you can use in vanilla to make things more comfortable for you.
Press t and type /time hoursperday [number] will change the total number of hours in a day. So instead of 24 it can be anything really. You'll still get shifting day/night cycle depending on your time of year but it will give you more to play with. It'll make those beds come a lot more in useful.
And then there's /time speed [number] (default 60), which sets how fast those hours pass. So the default length of a full day is like 45 minutes I think. So if you take that down to /time speed 46 I believe you get an hour IRL per in-game day. Again, won't change the day/cycle itself but it will give you more time to get stuff done. Then you can sleep through the night. I have historically done a lot of 1 hour IRL days, and they give you a nice extra buffer to play with, especially in the early game.