I have 2 lovely awase kimonos but it's unbearably hot where I am, so I was wondering if the nagajuban could be omitted entirely if a date-eri/kasane-eri is sewed on? Thanks in advance!

:D One-of-us-one-of-us :D (hi!)

Nothing forbids you to ditch juban!

In fact there is a term for that: 素袷 suawase (=wearing a lined kimono without an undergarment​).

To mimic the juban without actually wearing one, you would not use a dateeri but a another kimono item especially made to "fake" juban collar called an usotsuki-eri:

Tbh, I often use this trick, especially for indoors events where I know it will be (too) warm! I keep the hadajuban (or something similar like a slip), add a fake collar, and I am good to go.

Will that get a pass for formal events? no. Is it comfortable and allowing you to wear kimono in Western-heated-rooms? yes!

Now the thing is, if it's really really hot where you live, I would not advise wearing awase too often without proper foundation under it. Undergarments like juban are also there to protect kimono from sweat, so not wearing a juban could in the end damaged your kimono - especially if they are silk!

Koromogae (the change of clothes -lined/unlined- to better match the seasons) historically took place around April 1st in Japan. But it was not set in stone, and widely depended on where people lived!

If you can, dress accordingly for your climate :) I'd favor unlined hitoe (maybe you could alter your awase?), or even nice yukata dress-up as kimono.

This way you could still wear your kimono but with proper undergarments to protect them ;)