1. to know it's ilvl 50 you can either check the level where you found it or have someone with maphack look at it. i refuse to use mh so i just cube charms that i've found in areas that i know give it an ilvl > 50 but not an extremely high ilvl (act5 nm, act1 hell).
2. it doesn't matter. i have a lvl 2 mule that i keep all my pgems on and he's cubed a few skillers.
3. not sure of the exact value because each skiller is worth a little different, for example, a pally combat skills charm is worth quite a bit more than a pally defensive auras charm. for the charms that i don't need, i usually ask for around pul-um for them, but that's west ladder. if it has a second mod, it could be a lot more depending on what that mod is. my highest charm is a +1 traps +27 life and i traded that straight up for a 501 def three socket archon plate.