Look on the serial number plate (rear of the mower deck) on your mower. The full model name will be there. If you see the "9" in the name, you got the right blades. If not, you should return them and get the factory-recommended blades that match the ones that came with your mower. Look for part number 08720-VE2-001.
A small door/shutter that lets you choose to mulch or bag. It's a green lever on the rear of the mower deck. If you do not have one, you for sure do not have have a K9 mower, and need the different blades.
No. The washer is curved and has some spring built-in. Compare an old one to a new one. If they are the same shape, you can use the old one. Eventually, the old one will start to get flat (no spring) and should be replaced.
When working around the blades, you just don't want to take any chances. Disconnecting the spark plug cap is 100% effective against any possible accidental engine start.
Provided nothing else fell or or came apart (varies with models) just tighten it back securely. Not usually an issue.