How and why humidity and vapour pressure change as a parcel of air with an unchanging amount of water vapor rises, expands, and cools
The imortant point is that absolute humidity is defined as

mass of water per volume of air

So if the parcel of air expands, and still has the same water content, there is less mass of water per volume, hence the absolute humidity decreases.

Maybe most if not all of the question depends on this and is much more clear now?