Texture Mapping (1) and Custom Block (2) Issues

I have found two serious obstacles on MCreator 1.8.3 (1.12.2). I have seen others having similar problems, but those have mainly concerned older editions and are generally less specific, so I decided to ask about this version.

For one, at least for MCEdit 2.0.0-beta14, all custom blocks are mistaken for Vanilla ones (e.g. air and buttons). This is regardless of whether .schematic or .nbt is used. I also registered the blocks ('Configure Blocks') by their numerical ID's, i.e. XXX:X. If this is fixable with the current MCEdit, how can that be done? If not, which version (or, alternatively, other program) does not exhibit this problem?

The mobs also seemingly support only .java models, but these do not allow for texture mapping. How have you been able to assign a specific texture for each component of a custom entity despite this?

As said before, I have tried several different methods -- ultimately falling short on these two problems. I am clearly missing something.