Disney Death: Hiccup appears to be dead for a few minutes, and everyone starts to grieve him...and then Toothless wakes up, realizes why everyone's sad, and opens up his wings, revealing that, yes, Hiccup survived. Er, well, most of him...
For Want Of A Nail: Unlike in canon, the Red Death calls in its colony of dragons for back-up.
In Spite of a Nail: Despite the addition of the Madrigals, their dragons and the impact they have made on the story, the battle and defeat of the Red Death ends pretty much the same as it did in How to Train Your Dragon: the Red Death is destroyed and Hiccup survives missing a leg.
Major Injury Underreaction: Downplayed. When Mirabel sees the massive scars she had gained from the explosion, she gives a "hmm" and jokes about it not being the souvenir she was expecting. The fact that the wound had healed before she woke up with it probably helped.
Scars Are Forever: The burns Mirabel gets are so bad, while the last remaining arepa undoes most of the damage, it leaves her with noticeable burn scars.
