Hey Redroach, I'll try to answer your questions but for A and B you'll have to wait for an expert to pop by.
a) As I don't levelbomb yet myself this is just an uneducated guess...but I assume it's mostly a case of trimming out as perfectly and from as far away as possible and presetting the bombsight to the height and speed you plan to drop from, then juggling back and forth every few seconds to verify your values right up to the point where you drop. The higher and faster you go the bigger any deviation will affect accuracy ,so it's probably best to not practise at crazy heights and speeds.
b) Heck if I know...
c) If you have a textbox open it is reasonably easy to assess damage as you can enable messages that pop up for each system that gets damaged, many turn these off for a more authentic feeling though and then a wingman who can inspect your plane on the outside is the best there is.
For the parachute issue; press "alt-F2" after ejecting to get a 3rd person free camera that you can move around. You can also use this after landing to inspect possible damage. But be warned, you can't get back into the plane once enabled so don't ever use it for taxiing, while flying or when still in the middle of the runway.
d) This is caused by a server script that cleans up planes after they have landed. It ensures a minimum of clutter from landed or abandoned planes and thus keep performance reasonable.
Generally once you've landed just let the plane roll off the runway so it's out of the way and "escape" back to the menu. Soon as you come to a stop the "...has landed" will pop up and that's all that realy matters
A bug where your plane despawns shortly after takeoff is a side effect of this, sometimes you might bounce when taking off so the script will register it as a touchdown and remove your plane a little later.