I had a thought:
I've gone most of this time thinking that when the troll broke through the floor, Reynir froze for the few moments it probably took for Lalli to get in there, and didn't do anything at all -- maybe because what I was expecting him to do was to throw up a protective bubble, though I don't know whether the one he used against the ghosts would work against trolls.
But take a look at page 654. What's that attracting Lalli's attention? Isn't that Reynir's spirit dog there on the left of the page, first barking at Lalli and then turned back to the tank to lead him in the right direction?
Maybe Reynir did what Reynir does best: yell for help from the person most likely to be able to provide it. And that is a genuinely useful talent; if, in this case, fractionally late -- I was going to say fractionally too late, but it was in time to save Reynir and Kitty, and at least buy Tuuri some time.