When it comes to gerunds ya just gotta stop the sentance.
Accordingly, it is correct to say I can understand his wanting to go, but incorrect to say I can understand him wanting to go.
So just stop after the subject "him" ... "I can understand him ... " and then the "his wanting" becomes easy-peasy choice.
“we appreciate you being here.” becomes "we appreciate you,(pause) being here" ... when what you want to say is "We appreciate your being here."