Regarding Ewoks in ROTJ, I remember seeing something years ago, that Lucas originally wanted to make a planet of Wookies.
It could’ve been on a Discovery Channel program. However, since the Chewbacca costume was made of horse hair, and was extremely expensive, they had to change it completely. Make a planet of small creatures, using less expensive material. The name Ewok came from Economical Wookie = EWOK.
I don’t know if he ever actually considered it during the making of ROTJ, although that was my impression as well. He did however have Wookiees help destroy the DS in the first draft of ANH. So he might have planned to recycle this idea, only to change them to Ewoks for practical reasons like you mentioned. So Lucas did since the very beginning toy with the idea of a primitive culture helping the rebels take down the technologically superior Empire. So regardless of what one might think of the Ewoks as they turned out, the core idea has a very appropriate symbolism to it.