Yes.

No.

It is the maximum distance from an enemy unit at which it can declare a charge.

No. The only exception to this is when a unit pivots.

Yes, provided the unit does not end its movement 'on top' of another unit or within 1" of an enemy unit. When a unit wheels, those at its rear do not really follow a long, curving path, as it might appear when moving a large block of models on the table. In reality, they would take a more direct route, following the path of the front rank and resuming their formation behind it.

If there is a chance of the Skirmishers moving so that they are no longer an obstruction (if they declare a charge, for example), yes. Otherwise, no.

There are several reasons. Firstly, a Drilled unit that declares a charge whilst in Marching Column can freely redress its ranks to adopt Combat Order after determining its charge range, but before moving, thus allowing it to make a charge move.

Secondly, units that are obliged to declare a charge in certain circumstances (those that are Frenzied or Impetuous, for example) must do so even whilst in Marching Column. If they cannot make the charge move, they don't move at all and the charge is failed. This prevents Marching Column being used to avoid declaring compulsory charges.

Alternatively, there might be a psychological advantage to declaring a charge with a Marching Column. For example, the unit might cause Terror, or the charge target might already be fleeing.

Of course, it isn't easy to set up situations where such tactics can be used and therein lies the challenge.

If it is able to redress the ranks (i.e., if there is space for it to do so), no. A unit that is obliged to charge must endeavour to make use of any special rules it has in order to charge. The unit just really wants to charge, and it'll play this game without you if it has to!

No. Sometimes a charge isn't possible.

No.

No. As stated on page 124, no model can move further than twice its Movement characteristic. However, there are obvious exceptions to this, such as when a Marching Column marches, or when a dice roll determines a unit's movement.

Yes. The unit is considered to have moved onto the battlefield. Therefore, no model can be placed more than twice its Movement characteristic from the edge of the battlefield its unit enters from.