I would definitely add bearings on the top, not just for the threat of axes but when the disc collides with something force isn't just applied laterally. For example if the disc/blade hits an angled side of another robot the disc could be deflected downwards/upwards slightly even if you tear straight through the armour. In extreme cases this could bend your disc leading to it crashing into your chassis. In my opinion I'd rather let a bearing take the grunt and explode with the possibility this will jam your blade, than having the blade warp with the possibility of it crashing into and damaging your chassis. Even with bearings you can still have the blade warp however. It's mainly down to personal preference and learning from experience.

I assume from your CAD that you intend to mount the "saucepan" (That would be really cool to see) directly the weapon motor. This is probably a bad idea as when you collide with something alot of force is going to be transferred directly to your weapon shaft, possibly shearing the rotor spindle. If you look at spinners especially in the heavyweight class(You can see how Carbide does this in Episode 6 of robot wars, You can also see PP3D's build log on the forum which uses a large titanium spindle for the disc with the ETEK motor mounted behind it. I assume chain driven) they do not drive the weapon directly from the motor. You could move your motor off center and either chain or belt drive it to a central shaft which your outer weapon is connected to.