OP -- I'm glad you had a terrific vacation and all worked out well for you!
I do agree with others, though, that the bike trailer is highly unlikely to be rated for transportation tie-down. Most special needs strollers are not rated for transport tie-down, hence the requirement that they be folded on buses. While the lower framing of the bike trailer may be quite sturdy, it is designed to move either with or away from the crash, it is not designed to withstand the same type of force that a tied-down mobility device can withstand. Devices rated for tie-down during transport typically have welded areas creating extra sturdiness at that tie-down spot. The forces involved in a sudden stop, to say nothing of an accident, could bend/break/damage the framing of other equipment inappropriately tied down. I see nothing on the Wike site indicating what kind of safety testing is done on their trailers.
However, my biggest concern regarding a child remaining in a bike stroller while tied-down on a bus is with regard to the upper portion of the stroller. Bike trailers are generally designed to NOT tip over, therefore the upper portion does not have the sturdy all-around framing of the lower portion. In a bike trailer, the child rides extremely low to the ground, much lower than typical seat height on a bus. A child riding so low could easily have an adult fall onto/into the stroller if the adult loses his/her balance. I don't think that upper frame has been tested against a 200lb or 250lb person putting full weight on it. Add in the additional forces of a crash or roll-over bus accident, and an adult passenger could probably crush the top of that stroller right down on the child.
It is true that the bus drivers will most likely take a guest's word as to a mobility device being transport-ready. I am surprised they allowed any type of stroller device to be tied-down, as they are usually quite insistent against that practice. In this situation I think it was an usual enough piece of equipment that they weren't at all familiar with it.
Again, though, I am very glad your family had a great vacation without the stresses that came up during planning!