Thing with lairs is, you can't build camp fire in it or in its area and you can't sleep in it or in its area. However you can wait through night inside one without being attacked by boogeymen.
that because the bogeyman mechanic is tied to a separate thing that not tied to it being a site but if you the player are outside and alone.
being inside at night prevents the cackling to activating and the bogeyman to spawn. this is something that can be done anywhere and not tied to the site. if you find a cliff-side that has an overhanging ledge you could hide under it and be hidden from bogeymen or building a floor one tile above the ground can also work or freezing water above ground next to a wall in vanilla terms. the prevention of campfires probably comes from the fact players could build a temp campfire wall in a lair trapping the folks inside and or preventing them from attacking you.
Gonna guess waiting it out you mean mashing the . and , keys until the sun rises given you never fast travel.
back on the subject of abandoning adv made camp sites
claiming a site doesn't immediately give you ownership of the site there like a whole another mechanic tied to it from others staking claims and pulling off insurgences until there no one there to counter your claim (the adv camp meets this requirements of giving you control as there usually no one there who claimed it already standing there). abandoning these camps at this point would be saying "hey this person isn't defending their spot and someone else can take it."
hmm a different explanation would be abandonment is an Fort mode thing that fort mode players can currently do I guess one could attempt to Raze a camp to the ground but they typically don't have an ruin state as there nothing to trash in a camp. there also the bit where folks just move in to empty sites in worldgen and legends mode.
the thing that happens with companions in sites they originally came from probably due to some site zoning assigning them to a spot and returning to that area loads them in to that spot first.
it's also how one could zone companions into walled off areas on your camp and build a container for them.