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To answer this question you do and always have to specify a model, however you do not need to specify the textures within the model, that can be done in the blockstate file with the forge blockstate system. This allows for easy creation of basic blocks IE fully square ones. You only have to refer to the vanilla models and specify what texture(s) to use in the blockstate, allowing for simple blocks only needing one Json file. including the ItemBlock with the inventory variant.

JEI has a wiki describing how to create plugins for it on it's github page.

If I've understood your problem, maybe having a look at this could be helpful. This is a Thamcraft JEI plugin.

Okay, since I haven't seen anywhere exact solution for this I will write my solution I have found here just in case someone has the same problem. OnUsingTick as it has been explained above and in the other threads is quite bad for doing this NBT related stuff, but you can subscribe to the ClientTickEvent and detect from here using the Minecraft GameSettings if your key (keyBindingAttack/UseItem - for different mouse buttons) is being held and run the code from here. But keep in mind you will have to sync some stuff with server in order to work properly. I'm not saying that this is how it should be done, this is just how it works fine for me.

Don't trust everything you read on the internet kid. Especially of it's a magic mod that suposidly increases MC performance. There is a reason why Speiger has been banned from not only Forge, but many other major MC projects. Yes, Forge keeps some extra data about the textures in memory. This is to speed up rendering, lighting, and the like. There is a triad off of some extra memory usage for massive decreases in cpu usage. And at the end of the day, RAM is cheap, CPUs are not. Also, The guy who wrote the changes Forge does is now actually employed at Mojang rewriting their rendering engine. So i'd trust his code over Speiger's. The same goes for any other "performance" mod you see out there. There are good reasons they arnt officially part of Forge. Most of those mods break other mods, simply because the deep level of control that mods have over their own data and MC.