> there are two distinct sounds in the kana table for う and お.

Oh no, that totally escaped my feeble attention. Boy, do I feel sheepishly stupid now.

> Go ask a native to slowly sound out the individual mora

In fact, now that you point it out, even if I do that myself, it's obvious they are different: ko-u-cha, o-o-sa-ka!

Well, I've just been going about this all wrong, barking up the wrong tree.

In hindsight it now makes total sense that they wouldn't just use う as a marker to indicate that the previous お is long. Thats what ー is for; whereas う has a sound!

Ohohsaka, coacha: gonna practice that.