Not all chokes are removeable. If you do have a modular choke you'll damage your gun faster firing without a choke tube in place faster than by firing slugs through a full choke. They make thin wall choke tubes if you want to have an open choke with a modular choke system. From my admittedly 2nd hand knowledge rifled slugs through full choke is safe, but generally inaccurate.
Yup, I have shot slugs from choked barrels on many occasions and it works just fine. Some shotguns shoot rifled slugs a little better with a modified choke than a cylinder choke.
Yes they get a bit smaller at the end to reduce the spread.
Bullets are quite malleable and it’s normal for them to deform a bit during firing. Bullets for non-shotguns typically get forced down a bit in the forcing cone before being deformed to fit the grooves in the rifling.
“Rifled” lead shotgun slugs have these grooves around the outside that allow them to deform down to a smaller diameter. You can shoot them through a choked shotgun barrel just fine.