"Lots of variation and anatomical inconsistency" could totally be one of their features as a species.

Maybe some can stand on their rear legs, but others can't. Maybe some can curl themselves into a ball, but others can't. Maybe their face tendrils can expand and contract in ways that seem impossible. Maybe their body produces bone as needed, then reabsorbs it later, so sometimes they can fit into small spaces but other times they can't.

That's the fun thing about alien biology: it's alien. It doesn't need to follow the same rules as animals here on Earth. Just come up with a reason that sounds like it makes logical sense, whether or not an Earth animal could plausibly do it.