my project requires this plugin and Im tired of waiting months for you guys to get on top of things
Allegorithmic has never promised to support preview builds for UE4. They are not meant for real projects but for testing purposes only. That being said, I'd appreciate if they did do initial changes on github when first previews hit the launcher. Basically the reasoning why they don't do this has been that there may be some changes so they'd have to change the plugin multiple times.
If you must use preview builds or can't wait for the official plugin to arrive to marketplace, you can always try to compile the plugin yourself. Source code is supplied even with the marketplace version.
For example, I took the 4.11 plugin, placed it into my own UE4 4.12 c++ projects Plugins-folder and compiled it there. On 4.12, there's duplicate definition for one struct, but if you rename it on the substance plugin, it'll compile, work and package just fine (at least I didn't see any issues with it). If that's too much work, then maybe just stay on 4.11 and wait until things you find you must have are available?
There are 20 code plugins on UE4 marketplace and I don't think any of them support 4.12 at the moment, so please be patient, 4.12 just got released yesterday after all.