Hi Waxoff, You are mistaken. The upper cuff is not riveted to the lower cuff anywhere except the ankle pivots. Doing so wouldn't allow the boot to pivot forwards and would negate the entire concept Dalbello touts with their design. The ankle buckle is screwed to a post that protrudes though a hole in the cuff which is considerably larger than said post. This allows the cuff to freely move around the buckle.
While it may not give just as good as a walk mode as the lupo, it is significant. Regardless, removing the tongue should increase that ROM significantly. Perhaps the designers thought undoing your buckles to help enough, but then I would be concerned about foot movement.
To give you some context, I modified my Atomic Hawx 130 XTD to have removable Dalbello B tongues. With those boots in walk mode with the tongue in, they are stiffer than no tongue with the boot in ski mode.