I have objections for the same reason because Shieldwall it is not triggered ability but an activated ability. "At the start of combat" is only once so everything that happens there should be resolved before we goes to combat. In my opinion, there is a difference between triggered and activated ability.
Following what you wrote, since all unit fights through High Tide in "at the beginning of combat" I could still use this activated ability or any other activated ability.
I'm wrong?