48. does the ranger's hit and run power work like spring attack from 3rd edition (meaning, can you move, then attack, then move again)?
Just read what it says. A power that lets you do that either says shift, attack, and shift, or move, attack, and move, with the move not provoking an opportunity attack. There are lots of ranger powers that let you shift before or after an attack.
IIRC Spring Attack was a full-round action in 3e. In 4e, a power that lets you shift and then attack is a standard action, so you can still move away afterward. The Essentials melee ranger has stances that give free shifts and so let you replicate the effect.
Because full-round actions don't exist in 4e, you cannot literally replicate it. There are monsters with similar powers though, called Mobile Melee Attack, which is actually better because you still have a move action to play around with.
49. do all at-will powers increase in damage at level 21?
I haven't seen any that don't.
50. does prime shot affect all ranged powers?
Yes. It says ranged attacks without any additional restrictions. If you're multiclassed, or using a non-class power (such as throwing an axe as a ranged basic attack) Prime Shot still applies.
Note that it applies to both rangers and warlocks, despite one using ranged weapons and the other using spells.
51. is there any way to change the frequency a power can be used (like making an encounter power usable at-will)?
Not directly, but the demigod epic destiny causes you to recharge an encounter power whenever you have none left, so in effect you never have to use an at-will. This makes it one of the most popular EDs in the game.
The Fey Charge feat lets you not use up Fey Step if you hit with the attack. Power Strike is supposedly an encounter power, but you get more uses with levels. There's an item that lets you use a channel divinity more than once.