1/ Engineers are indeed qualified to conduct engine runs, with or without crew present. Certain engineers are indeed qualified to taxi the a/c under power rather than with a pushback/tow tug. some engine runs I have witnessed go outside the normal flight crew operations, so we are consigned to the jumpseat for 10 minutes or in the cabin for a cup of tea.
2/ An aircraft with engines running has anti collision lights on, if you remember that part, and also with engines running, a qualified engineer will be on the headset so all vehicles should not approach the aircraft. Also a post departure engine run such as this, all catering will already be on board.
3/ Would you argue with a hairy engineer, whom you have pulled away from his mug of tea or the afternoon football. After all, the aircraft if subjected to what you are insinuating, still needs to be pushed back.
4/ What would be different from being in the cockpit or in the cabin when smoke from a faulty motor puts some smoke in the cabin, make you own assesment quicker than ahving it relayed to you on the flight deck.