Whirring noise when cold. Doesn't want to select gears properly until warm. Whirring then goes away. Noticed drip underneath the gearbox but does not feel like oil unless gearbox oil is very watery.

... indeed ATF is low viscosity.

To sum it up, your transmission is grinding its bearings with low fluid caused by leak.

This type of extreme damage may not be fixeable with a rebuild kit.