900mm Hob with 850mm Cut out over 800mm Base Unit

Ive done this in the past and cut out 50mm below the hob in case the hob get warm. Then silicone seal the cut out. Good luck

Most hobs don't have great depth to them and yes I have come across this in projects many times with no issues. And yes make sure you silicone around the hob, water/food will eventually rust the hob underneath.

Yes this is common practice assuming the kitchen has been planned for such, ie the cut out's will not involve the removal of any other support for mechanism's like draw rails etc also subject to location restrengthening of the Unit can be required, there are usually installation guidelines for this included with the appliance General rule, cut out the minimum required for clearances to retain as much strength to the unit as possible, and take your time to mark out the position accurately to insure a good fit

Cutting an aperture to accommodate the hob will not cause any structural problems with the base unit. I would recommend sealing the cut edges with aluminium tape which will give some thermal insulation as well as repelling any moisture.

Yes you can its what you need to do sometime. You'll have no problems with carcases