Jack - the point of getting out of your seat would be to hopefully ELIMINATE the engine leak (not PROVE the tank leak) as if it is significant you will have a good chance of seeing it (even at night, when there is nearly always
some background light - I speak from a few years of flying visual identifications on unlit targets. Only once out of many was I unable to see something). Of course, if you are in a Tristar or...............
'Max fuel imbalance' would be determined by controllability rather than manufacturers' limits if fuel quantity becomes a life-threatening issue.