You keep the Drow race's range on Darkvision. The rules say you retain your racial features, but can't use special senses unless your new form also has them:

You retain the benefit of any features from your class, race, or other source and can use them if the new form is physically capable of doing so. However, you can't use any of your Special senses, such as Darkvision, unless your new form also has that sense.

Emphasis mine.

You retain your race's Darkvision, and so you retain Darkvision 120'. If you wild shape into a beast that does not have Darkvision, you can't use your Special Sense but you have still retained it. If you wild shape into a beast that does have Darkvision, though, then you can use it. And since you retained the feature from your race, you use your race-granted version of it, which has the longer range.

No, the Giant Elk is given only one Action (and no Bonus Actions), and thus can only choose to do one of the available actions.

The text for Charge reads (MM, pp325)

If the elk moves at least 20 feet straight towards a target and then hits it with a ram attack...

Your Actions(and specifically those for Attacking) are as follows (MM, pp325)

1. Ram
2. Hooves

The Charge means you are choosing the Ram Attack.

The next round, if the creature is still prone, you may attack with the Hooves. Otherwise, it's just another Ram attack without the charge (unless you take the OA to move away and run again at them.)

And as Andras says, there ARE beasts like the Elephant (MM, pp322), that have viable Bonus Actions available to them.

Trampling Charge. If the elephant moves at least 20 feet straight toward a creature and then hits it with a gore attack on the same turn, that target must succeed on a DC 12 Strength saving throw or be knocked prone. If the target is prone, the elephant can make one stomp attack against it as a bonus action.

The (Giant) Elk, however, is not one of them.