The location of the egg in the ceiling is very easy to explain. Too easy, actually, and it all comes down to space travel energy usage and conservation.
What's the first thing that happens when one of these spacecraft reaches its destination? It automatically prepares itself for human occupancy. Lights come on, monitors blink to life and systems generally fire up. Up until someone is actually around to use them, all of these systems are in 'sleep' mode simply because there is no use for them to be active during a flight when the entire crew are in hypersleep for months on end. Leaving all the lights burning and keeping monitors on serves no purpose when there's no one around. As soon as the crew are awakened, these systems are automatically activated. It's a sensible way to conserve not only energy, but prevent component failure as well. It's good engineering when resources can sometimes require calculation to the last watt/pound/whatever. Bottom line is, space craft can't afford to waste energy.
This would also apply to whatever artificial gravity systems are in use aboard the Nostromo/Sulaco. There's no point in expending energy on keeping people upright on their feet when they're safely cocooned in a sleeping pod for months on end. So the artificial gravity systems are also powered down. Let's just give an arbitrary value of power down to be in the range of 85-90% - just enough remains for stationery objects to stay where they are, permit friction to act upon them to keep them in place and allow their inertial frame of reference do the rest of the work - and substantially enough of a reduction to be energy conserving. Just prior to everyone waking up, the artificial gravity is automatically raised to 100% along with the rest of the ship's systems. Sounds about right.
The crew then do whatever it is they gotta do and all the systems are active to accommodate their activities, including artificial gravity. Makes sense.
When the crew go to rest in their hypersleep beds again, everything powers down to conserve energy. All monitors and lights are dimmed and other systems that no longer needed to be supervised by humans can be set into 'sleep' mode. This goes for the artificial gravity systems as well - which conceivably could be very energy intensive. Why have them on full when everyone's asleep? Power them down and conserve energy. Makes that long sleep easier, too when less gravity is in effect.*
At a reduced gravity, say 15%, the mobile pre-larva is able to move about more freely when everyone is tucked away in beds. It can attach itself to any surface under reduced gravity conditions. It makes sense to locate itself as close to the hypersleep pods as possible. How does it know to do this? Well, let's just say there's a lot about Alien perception that's always been debatable. Maybe Aliens are like sharks and can detect bioelectricity? Who knows?
So anyway, the larva attaches itself to the ceiling because, well, there's simply less gravity and any surface is ideal. The small organism attaches itself in place, sends out tendrils and starts to form an egg. The use of its acidic bodily fluid dissolves components of the ship it is attached to and it grows an outer covering. Eventually it becomes a fully grown Alien egg, complete with an 8 legged facehugger within.
Mind you, that 15% gravity is still in effect as the egg grows and as it increases in size, it eventually gets to a point where even that reduced artificial gravity exerts enough force on the mass of the egg, especially as it nears full size. Eventually that 15% gravity is enough to dislodge the contents and force the egg to open. The facehugger then quite literally falls out onto the floor without the need for anyone approaching it.
Well, what's it going to do, now?
Go looking for a host, of course. You know the rest from here. Especially if the Queen has dropped 2 (or more) proto-facehuggers on the Sulaco deck during its confrontation with Ripley.
-Windebieste.
*notwithstanding the fact that long periods of low gravity do have adverse affects on human health. No doubt WY have contractual clauses and disclaimers in place for that. lol.