From my experience it is virtually no different to normal checkin as with your question:
1) People will respond to the DG questions exactly the same and it does not matter if they are written or verbal. The checkin agent and airline has to go on the honesty of the pax with either method. The back up as approved by DOTARS is that all hand carry is screened and all checked baggage will be 100% screened very soon.
2) The ID check is by having to use the credit card that was used to purchase the ticket to obtain the boarding pass at a kiosk. With normal checkin you can book and pay for a ticket in your name, checkin and go to the gate then give the boarding pass to your mate who is seeing you off. This can happen with every domestic airline in Australia now.
The backup as okayed by DOTARS is that every person and their baggage is screened before getting on the plane.
3) As John says, hand carry is supposed to be checked for size and weight at the gate. Thats why every airline has or should have test units at every gate.
The only way to be 100% certain of who is travelling is to check ID at the gate like all US and UK airlines do for international flights. DOTARS has determined that the correct name/pax is the lowest risk because whoever and their baggage gets on the flight has been screened.
Receiving a printed barcode boarding pass at home is the same as above. Again, you have to be screened to get on the plane and all bags have to go through the same system no matter which way you checked in.
Web and kiosk checkin is mainly designed for the business traveller who have no checked in baggage.