To me, however,
the vowel in 'boot' is clearly _longer_ (different in quantity) from the
one in 'book'. Native English speakers don't seem to notice that difference
- yet, to my ears at least, they produce it accurately every time they utter
the words.


Wow, you nailed the other _big_ thing that's made it very hard to master pronunciation of ON/Icelandic!  In English we are never taught the concept of long vs. short vowels in terms of quantity.  Every schoolchild when learning to read is taught "long" vowels (cane) vs. "short" vowels (can), but in this case it's a misuse of the terms, since it really means two totally different vowels.  We are actually unaware, as you point out, that the vowels take longer to sound.  And when I read that in many languages one has to pronounce the vowels for a longer or shorter period of time, I was mystified.  It took me years to understand that concept, let alone hear the difference.

Tim