Regarding question 2:
If the unit is a vehicle, you can use Mando_Chaser, found here:
http://www.ofpec.com/ed_depot/index.php?action=details&id=387&game=ArmAYou could use a gamelogic for the unit to be chased, and the chaser would move in a straight line to position of the gamelogic at the speed specified.
If the unit is a man, you cannot specify an exact speed. Men move at the speeds of their animations (walk, run, sprint) and stance (prone, crouch, erect). And their AI will make them wander off course.
However, you could write a script where you pass in direction, destination position, and animation name. The function would then face the unit in the desired direction (dowatch or setdir), and then within a loop, you repeat the animation (walk forward for example). The loop repeats until the distance between the unit and destination is close to zero. You would have to also know how long it takes a unit to complete the animation, so your loop would sleep that long (otherwise a zillion playmoves would stack up, when you only needed a few playmoves to reach your destination). This is very doable. And would be useful for cutscenes and very controlled scripted situations. However, in dynamic situations it can cause trouble because a playmove or switchmove can force a unit to clip through an object that blocks his path. So if a tank is parked between a unit and his destination, he will bump into it or clip through it and look stupid.