I am also pulling my hair out over the best/easiest way to do this..
I have however, done a TON of ground work, so i may as well share it here and hope it helps someone I have yet to find my perfect solution still.. but this set of tools definitely has the potential to get the job done..
So far, the available options I have found are:
3DPrint.app (MineOS application to design custom blocks in layers, ran directly from OC computer in MC) I translated a copy of it to english here: MineOS - 3DPrint.app - Eng-US Patch
OPL's Minecraft Model Creator - Decent program, not something I used much though, i prefer something with a little more functionality (Even copy/paste/duplicate would have been nice, but it didnt seem to work right for me. Also, not sure if it was just on my end.. but the undo/redo features did absolutely nothing...)
MrCrayfish's Model Creator - Very well designed, haven't had enough time to give it any effort but the interface design and controls could not be easier!
MagicaVoxel - Visual Voxel Based Model Creator is a very well build designer and comes with a super simple interface and more than enough examples
Then Use: MagicaVoxel Vox to OC 3DPrinter converter to convert your .vox files afterwards to MC JSON so you can easily import them in game.
Converting .Schematic files to 3ds (Possible Solution, I haven't and won't be trying this as I am more interested in creating custom blocks from scratch)
NovaSkin has a damn near idiot-proof model/texture designer WepApp (7 Mins was enough time to make a 3 custom textured blocks exactly how i wanted them)
However, even if you find no use for the online designer itself.. The website has one of the most complete and easy to navigate set of models/textures in the style of a massive Repo/Database.
And a few others.. "not worth mentioning". However I will be sure to come back and update this post if i find any more "winners" haha. All I have for now!
Good luck and happy modding!
- [TiC] Detox_GG