IIRC, the phy attach soft body box/sphere/capsule commands are needed only when you want multiple soft bodies to collide with each other. So, if you only have one soft body, you don't have to use them. They form like an invisible "cage" for collision between soft bodies, otherwise collision won't happen.
Unfortunately, the answer for both the other two questions is no. There's no way to detach them apart from deleting the soft body. Additionally, whenever I've tried to attach a soft body to a rigid body through a fixed joint, the soft body explodes.
Theory is when you know something, but it doesn't work. Practice is when something works, but you don't know why.
Programmers combine theory and practice: Nothing works and they don't know why.
