Dealer Kersti Clark was kind enough to immediately email us photographs of all the stickers and plaques to be found, including one that notes that the trailer is manufactured to ANSI standards, which should certainly address any US requirements about construction quality (ironically, at least two of the best made
fiberglass trailers are from Canada anyway).
It's really nice to get confirmation from someone who does work at the Border that what I was told about the requirements is correct--no DOT sticker needed.
Also, thank you very much for the suggestion on which route to take heading out... the Lighthouse Marine Park in Point Roberts sounds much more appealing than some of the other crossing points, especially if the lines are shorter.
Val