Re the speed. The ALS track shows that this sea jalopy was making a little over 6 knots at the time of impact. A 5kt “no wake” speed would not have made a difference. Modern container ships are mind bogglingly huge.
You hit the nail on the head with the observation about the lack of dolphins though. Maryland didn’t want to pay for them, and things were fine for 47 years, right up until they weren’t. You can bet any replacement bridge will sport dolphins or other features, plus a more robust (and expensive) span design.
I initially read 7kts, but you’re correct, 6,5,4, wouldn’t have made much difference, given the weight in motion.
Tug assist should have been mandatory for all large commercial vessels to get past that bridge.
In Miami, cruise ships have their own canal, all others use pilot boats.