Not all areas need to be sealed. I would not usually seal a prop and if I did it would be with a gloss coat then weatherd down to a semi gloss.
As for Acrylics coming of if not sealed.......its all in the prep and cure time. If you prep and prime your surfaces properly and then allow the paint proper cure time between coats, applying washes, decals and general weathering you should have no issues with paint coming off the actual model unless it is over handled drastically.
This is why I usually have 3 -4 builds on the go at any given time, which are usally different subject types. I'll have an aircraft, some armour and maybe a car all on the go at the same time. I'll let painted parts sit for 3-4 days before I handle them again, while waiting those few days I just work on one of my other builds. It also helps from getting bored with a single build and single subject type.
On the down side It takes alot longer to finish a single subject that way.