1. Not sure what you're asking exactly but here goes. I assume you're using spiderlegs to try to balance a limb over structure and they keep moving on you in unpredictable ways. All pieces have a center of gravity (cog) and with a spiderleg setup you are setting multiple slings on either side of it giving a much better control over a single sling pic, but the problem remains that the system will adjust itself so that the cog is directly under the hook. You can try to get it right by tensioning the tip side so it bends up hard when tensioned which means you have control over the tip (usually the part you don't want to swing down), but it leaves a lot to be desired.
For me at least spiderlegs are a slight step up from a single line in control, but they aren't good enough for rigging leads over houses. For that i greatly prefer 2 lines since it gives so much more control. It sounded like you recently bought a grcs, and so you'll want to use that on the tips then another rigging device for the butt. While you can crank leads up just like you could with a crane, it's far easier and quicker to simply swing them, so rather than notching it to fold up, notch it towards the open area. Come up on the tips to take the weight from the limb, then after doing the cuts and climbing to a safe spot you can have the ground guys initiate movement with a tag line (tied on the tip obviously) pulling it to the open area. When the hinge finally breaks the butt will drop a bit as the butt line takes the load, and the limb can be manipulated in the air to adjust how and where you land it. Here's a terrible picture of this setup set from the ground to pick this limb off of the roof.
View attachment 126665
2. The actual formula for force of a falling object is F = ma, m being mass and a being acceleration, which is 9.8 meters (~32 feet) per second per second
on earth. So for every second it falls it's going faster and faster, constantly accelerating until air resistance accelerates it fast enough to where the forces equal out reaching terminal velocity. To fully explain these relationships requires calculus, but my point is your linear formula isn't this. The longer an object falls the forces go up exponentially, so i wouldn't use that at all. Just know that the longer it falls the worse it is, and it's gonna be a multiple of the weight so error on the small side when negative rigging, and don't do it unless you have to.
3. Bend radius is very important in rigging, it determines how fast your ropes wear out or can even snap them. When a rope is bent while under load, the fibers closest to the sheave go from being under tension to under compression as the load is taken up by the outer fibers. The heavier the load and the sharper the radius compared the more you'll stress the outer fibers, which is what wears ropes out and causes cut lines. 3/4 line implies very very heavy pieces, so using small blocks/ rigging rings with large rope is simply inappropriate since your efficiency is going to drop so much that you might as well use smaller ropes correctly.