why you felt it necessary to mix ECN-2 instead of just using readily available C-41 chemistry
Because ECN-2 is the correct process for this film. (For those who may not be aware, ECN-2 uses CD-3 as the color developing component while C-41 uses CD-4.) As much as I like to scratch mix and match and find what materials CAN do in addition to what they are designed to do (as well as get away cheap where I can), I am not into the cross processing thing.
However, many seem to feel that running the Vision 3 films in C-41 works just fine. It is my sneaking suspicion, that if we could get under the hood we would find that Portra, Ektar, Vision3, 400 Max, Gold 200 and maybe even T-Max and the latest round of Tri-X are made of mixes and matches of the same basic components with the dyes and sensitizers tweaked. They are more similar than different. This being one of the key reasons Ektachrome went away - because it didn't use those same components.
Not saying they are the same films. Quite to the contrary: they all have their characteristics. I have no direct or inside knowledge, of course, but I think that using a small set of components in various optimized mixes is what keeps the films available.
In addition, I think that this same idea is what makes processing Vision 3 films in C-41 work out OK. By the same token, processing C-41 films in ECN-2 might work out OK, too. Haven't tried and don't plan to.
But a key thing to remember is that in any color film, the developer in oxidised form is half of the color dye. You don't get the same chromophore from CD-4 as you get from CD-3. But again, the dye components may have been engineered that either CD-3 or CD-4 are acceptable. IDK.
If you do the hybrid thing, it won't matter.
And this film shines for hybrid use. It was intended to be scanned because that is how movies are made today, well recently anyway.
printing to RA-4 with standard times, the negatives will be of low contrast
As I mentioned above, a 10% extension of development time, with constant agitation, gets a usable contrast. YMMV. I will admit that my biggest use of this film is a hybrid worklflow. As much as I like to, I do not have bunches of time to set up and print RA-4 - or anything else for that matter. Most (too much?) of my available time is spent learning and researching the various film processes. Sometimes Shutterfly is just the easy way out!
a positive print out the the theaters
I have thought about getting some ECP to make slides.