A negative drive train MOI is physically not possible as you say, but mathematically it is if the Roller/hub MOI is too high. The MOI wizard calculates the total MOI and subtracts the Roller/hub MOI to get the drive train MOI. 

Higher power in 5th than in 4th is another indication that your MOI is set too high. In 5th the rollers accelerate faster than in 4th, even with the same engine RPM sweep rate, so the proportion of power/torque coming from the MOI is higher in 5th than in 4th (which has higher load cell reading). This is actually a way to test the MOI setting. With correct MOI you should get very similar results in 4th compared to 5th. I say very similar because the MOI is not actually the same in 4th and 5th, due to the MOI of the engine and the gear ratio being different. This effect is very small, but nice to know.

Ok, so I need to rework my MOI using the YouTube video. 

thanks for the guidance

Measuring the MOI is sometimes tricky. The engine may behave differently when ramping up compared to ramping down, it may heat soak a bit (turbo cars) etc. You may try a few different ways to get a feel for it (ramp up then ramp down, steady state vs sweep, sweep in different gears etc). 

The MOI wizard is nice but needs little tuning on the brake % so you get a noticeable difference in the retardation time between the two runs, but not so much that the second run goes very fast (too little data). So good to test this a bit and get a setting that fits your setup. 

Of course you don't do all this stuff for every car. But good to do once or twice to get a feel for it.