changing direction

if bosh is rotating really fast, is it possible to turn/stop his direction with a fling/quirk thingy?.. or will it simply kill him?

Strictly speaking, anything is possible, as long as you can plan what you're going to do to a certain extent, and with enough patience.

On a less general note, you can use the nose for some pretty powerful stuff, if that's what you're looking for.

okay... well now i was doing hand flings.. and came to a point where it looks very much like a hand fling but bosh's hands is at the same position as when he isnt moving... could that be a weak hand fling or is it something else?

It's weak.

the speed at which you can take a certain turn is limited only by how much force you can exert on a set of contact points without them snapping past the point where he would die.

i know that doesn't really help much but anyway.

A mixture of both g-wells and contact point collision is good for changing direction. Exactly where to put them would take quite a bit of some time to explain.
Another good way is to try some trial-and-error until you find a super-strong well that serves as an anomalyLOLSEEWATIDIDTHERE? from normal g-wells

A quick change in direction, like anything else, is a trick, and needs to be mastered in a way, just like flings. It's something I always aim to do in my tracks, so it's possible

This is the vaguest question i've ever heard about line rider ._.

Sure. Some flings are much harder with a lot of rotation though (like feetflings and tailflings).