Neg decoupling means you were putting out more power for the same HR in the 2nd half of the ride. Thats good
The decoupling number shown above the interval on the ride timeline chart is not calculated the same way as decoupling for the whole ride. Its just the efficiency for the 1st half (avg watts / avg HR) compared to the 2nd. The whole ride decoupling looks at each minute of the ride, calculates and adjusts for HR lag, excludes not moving much minutes etc. to get a better number.
Outdoor is the thing for Z2 rides! But I can ride outdoors all year round where I live so I never do trainer rides. I tried that 12 years ago and hated it. Got my coach (I had one then) to take all that out the program!