I don’t have the X1850, but…
#1 is a known glitch on the x1800 and x1850 seemingly related to the sweep knob deadzones in the firmware not being quite wide enough. Lot of variation in these pots, it seems.
#2 seems like an issue with the pins in the headphone jack needing to be bent back to their proper shape. They’re pretty thin and springy. It shouldn’t be like this from the factory I would think.
#3 - If the cue is not the same volume as that channel wide open with the mix knob at 12 o’clock, that would seem to be a bad pot. Granted, if you’re expecting a cue-mix knob like on a Xone 92 or 4D, then this is obviously a different function than that even when it’s working right.
#4 - Did you have Instant On when checking this?
As for my experience with the X1800s, both of my mixers have done a little of #1 and only one of them right now still does any that I notice. The new firmware may have actually made that one worse in that regard and the other better. One of my mixers had a slight amount of #2 (the other mixer, actually) with one particular pair of headphones, but seems to not occur often. That mixer’s hooked up right now and using a pair of HD580s isn’t having any drop-outs. It might have been just a weird, fat Neutrik right angle connector that was causing it. And I have had none of #3 or #4.