I'm curious how "modable" is the final product made my Esenthel by end-users? ie Are they able to change and access the models, textures, etc.?

You have control over that. Final product release can be fully encrypted or totally open, or anything in between.

Oh. Very nice! Wasn't expecting that. lol. Every other engine it seems is completely closed off.

Sorry, I am curious what engines you are thinking of?