Mr. Shahar,  The legs on pianos with the Ampico which I have encountered
have been secured with machine bolts with a square head with a slot.
These bolts, as well as slot-head wood screws, can become very tight.
For this reason my driver of choice is a 3/8" ratchet with a slot bit on
an extension.  This provides increased torque and control over a standard
screwdriver.

That being said, I am unclear on your dilemma.  "The screw will not come
out by gravity" means that the screw is turning but not backing out.
This is not a result of a bent screw but rather of a screw whose threads
are not gripping any wood, if indeed it is a wood screw.  If it is a
machine bolt, then the collar in which the bolt fastens has become loose
from the case and is spinning as the bolt head is turned.

As most double-leg piano legs have two bolts securing them, and are not
keyed to the case, the other bolt could be removed and the leg forcefully
pulled away pulling out the 'loose- threaded' fastener.  (The piano
would have to be supported by a jack, or be its side.)

Regards,

Jon Page,   piano technician
Harwich Port, Cape Cod, Mass.
http://www.stanwoodpiano.com/