The object raised to that height has accumulated energy in form of potential energy, for the only reason of being at certain height in a gravitational field.
- The energy of lifting gets accumulated in the body, as long as there's the gravitational field.
- That energy can be released in form of kinetic energy, when you liberate the object and you let it fall. Those are the only possibilities: kinetic energy or potential energy, if we don't consider heat or chemical reactions...
- If the object is at minimum height, its potential energy is irrelevant. That agrees with the fact that the potential reference is arbitrary. We only care about differences of potential (here differences of height), that's the only meaningful thing. That difference in energies is what can become K.E.
- If the gravitational field disappears, so does the P.E. and consequently the object can remain in that position until something changes.
Now part 2 (next time try to separate questions, see how deep they can be ;) )
Yes, KE is frame-dependent, but that doesn't matter because, again, only changes in velocity do matter, not "absolute" velocity, if such thing existed. For example, collisions can be seen from many points of view, and all of them get the same results. KE is different for each observer, both before and after the collision. The variation of energy and momentum IS however the same afterwards, and that's what really matters.
I hope it's clear.