Deelybobbers and Diphthongs

In Devanagari, vowel symbols attach to a consonant (e.g. क - k) like this:

कि  -  ki                      की  -  ki:

के   -  ke                  

क   -  ka                      का  -  ka:

को  -  ko

कु   -  ku                      कू  -  ku:

What is cool about this is how symmetrical it is. In the upper vowels and the lower vowels there is a length distinction (mostly lost in modern Nepali pronunciation but preserved in spelling) - ki vs. ki:, ku vs. ku:, ka vs. ka:. The symbol that describes the upper vowel in the long is a reversal of the short: कि/की, कु/कू.

However, the 'deelybobber vowels' don't have length distinctions. The deelybobber vowels are mid-vowels, /e/ and /o/, and their Devanagari symbols look like deelybobbers: के and को . This is a very scientific term that I just made up. The Nepali term for it is eklaakha for one and dolaakha for two

The dolaakha are used for the two written Nepali diphthongs:

कै   -   kai

कौ  -   kau

We might expect, given the symmetry, that कै would represent /ke:/ and कौ would represent /ko:/. Instead, they represent diphthongs that contain neither vowel. However, the trajectory of the mouth as it creates these two diphthongs passes through the vowel in question: कै starts at /a/ and moves up and forward (passing /e/ along the way), and कौ starts at /a/ and moves up and backward (passing /o/ along the way).

Again, weirdly symmetrical. I wonder if this suggests a historical vowel shift in the middle vowels, which used to contain long mid-vowels that have since been diphthongized.

[Note: I've simplified the /a/ vowels, which I'm pretty sure are two fairly different vowels but that have been explained to me as 'reduced' and 'full' versions of the same thing. Also, I'm pretty sure 95% of the people reading this have no idea what I'm talking about, and the other 5% are laughing about how wrong it is.]