About Serial problems in user screen It is obscure for me. I'll try to replicate. Maybe serial connection introduces a delay time and/or has conflicts with your current arduino attached sensors/devices.
About characters, you can use createChar (this is not a MENWIZ char, it is a LiquidCrystal method instead, look at menwiz.cpp). Be carefull: it may conflict with MEWNWIZ, as it also create characters. Use char indexes higher than 3.
The user display elapsed time (the idle time before user screen appears) is the last parameter of addusrscreen. But perhaps I did not uderstand your problem.
If you refer to rapidly changes to values in user screen ... there are no ways other than to introduce a delay ... but it will slow down the interface responsivity. Try values around max 100 millis ... It sound quite strange as in my experience sensors values never changed at such a dramatic pace to let them unreadable on a LCD ... You can also and preferably set a user software implemented delay time before refresh values: store the latest read time and let the value be refreshed after that value (I suggest 500 millis).