This setting can be changed in real time via the spinner in the Movement Controls mini-floater ("View" -> "Movement Controls").
As for animations height auto-correction, it's the very reason of the
@adjustheight RestrainedLove command I designed and implemented (and that Marine adopted, so it's also in all recent RLV/RLVa viewers now). Scripted items using this command can auto-adjust the animations they play on your avatar (example: Cool Collar, Cool Cuffs).
I don't intend to make the AvatarOffsetZ a persistent setting (since it must be changed based on your avatar's shape and shoes, so it changes pretty much each time you change your outfit), but you could make it persistent on your own installation (search for this setting in app_settings/settings.xml and change the boolean from 0 to 1 under "<key>Persist</key>").