An Un-Calibrated Centrifuge

I'm pretty sure this was the last of the main series that I read as a kid. I wish the series would get back on track to stopping the Yeerk invasion. It feels like a long time since we've had a book that deals with that.

Questions under the cut.

I don't understand the term vecol. Is Mertil a vecol because he can't morph and that's compounded by the loss of his tail? Or is he only a vecol because of the loss of his tail? It's not super clear by usage alone (also neither really makes sense from everything we know about Andalite society...). Also the book didn't age well in its handling of disabilities and Ax's sudden ableism.

Why is the sky inside the bunker blue (like Earth) and not red/gold (like the Andalite home world)?

Why doesn't Gafinilan morph human to go into the backyard? There's a force field around the house but no hologram... anyone peeking in the backyard could see him.