It's possible.
Say, you have a planet in variable pPlanet1 (the same works for otheres, such as ships and asteroid fields, remember, E5/6 with the Kessok/Cardassian conversation?), then you can make it not collidable through:
pPlanet1.SetCollisionsOn(0)
There is another way, through the ProximityManager (more on this in a moment):
pSet.GetProximityManager().RemoveObject(pPlanet1)
I think they mean the instance, since AddObject works that way (RemoveObject doesn't appear to be used).
Be sure to create the proximity manager, via pSet.SetProximityManagerActive(1) it's possible that this is already present in your system.
Number 2 will have to be done through a ProximityCheck, look through scripts/Maelstrom/Episode6/E6M3/E6M3.py and E6M5.py, for the function def CreateProximityCheck(pPosition).
You can also look in MissionLib.py, but I think you'll find that the above scripts will do what you want, but be sure to create your own Event Type (the ET_* value).
If you want to have a moving, uncollidable, damaging, object, then you can attach the thing returned by App.ProximityCheck_Create to said object through pMovingThing.AttachObject(pProximityCheck).
Then, in the function you registed the ET_* to (you did do that, right?) you get the object that the proximitycheck found through App.ShipClass_Cast(pEvent.GetDestination())
And if the result is not None, then you can do this:
pHull = pShip.GetHull()
pShip.DamageSystem(pHull, <<YourDamage>>)
Where <<YourDamage>> is a number between 0.0 and some maximum damage.
This assumes that the result of App.ShipClass_Cast(...) is stored in a variable called pShip
Also, be sure to put at the bottom of your trigger function pObject.CallNextHandler(pEvent) or any futher events may not be triggered.
If you have any specific problems for the above, then please ask, it's just currently a bit too late for me to provide examples for all, and those may not be entirely usefull to learn from anyway.
For your 3rd question, I would think that the first number is the scale (since planet nifs loaded as ships are rather small), so if your planet model has a size of 241 and you put in the number 100, then the resulting model would be 24100 "units" in size.
Last question, I think increasing the SetSphereRadius of the backdrop could do this, I suggest you search the BCS forums for this, since it originated there.