When you fly upside down, you don't feel either the opposite of gravity or weightlessness.  My experience is that you are still pulled toward the earth.  I assume when the cabin is depressurised, the effect would be greater.

Don't undo your seatbelt!

Same as if you roll your SUV and are hanging by the seatbelt.

Gravity still acts towards the centre of the earth no matter which way up an aircraft flies so when flying upside down your entire weight is supported by the harness. This would be very uncomfortable in an airliner because only a simple lap strap is provided so your upper torso and legs will hang down (or up from your point of view) and fold around your waist. For this reason aerobatic aircraft are fitted with over-the-shoulder harnesses so when flying inverted all of your weight is supported by the shoulder straps.

At the point of landing aircraft normally experience a slight increase in g force as the pilot arrests the rate of descent. Normally this feels like being pressed down in to the seat but when landing inverted it will feel like you are being pressed in to the shoulder harness.