to insert the pin from below.
Considering the height of the summing lever/steering valve and it's associated cowling, it might be difficult for some people to reach up and locate the hole if the hole was above the assembly. On our tows in Australia, we would inspect the pin (and spring-loaded balls), insert the pin then pull down to test the locking abilities.
747 tow bars have two shear pins. If both break, yes, you can drive away quickly. If one shear pin breaks, and the tug driver is not driving at reckless speeds around corners, then gentle braking should bring the aircraft to a halt (with the aircraft still attached).
Tug driver communicates with tower. Brake rider on board may be monitoring the frequency but doesn't get involved.
Comms between flight deck and ground crew through hand signals.
We had either headsets with long leads (sometimes plugged into the tug intercom system depending on where we were riding on the tug) or wireless headsets..
plus walkie talkies and hand signals as back up.
Comms were required between the people downstairs and upstairs in case there was an urgent issue, such as an APU fire during the tow.