If I would be you I`d not use both Maxwell and Thea at the same time. There is no proof they are 100% compatible.
I decided to try Thea this morning. Here is what I found out so far:
Maxwell was rendering a scene just fine if I hit the render button once but if I made any changes to the model at all then hit render again Maxwell would just hang on the render forever and not process the image at all. A restart was the only way to get it to render. Then I uninstalled Vray beta from form.z and Maxwell suddenly started working fine again. However, I was still faced with insanely long render times.
Then along came Thea which I installed this morning (demo). I played with it for a while and found it interesting. Then I tried to open one of my recent models that was set up to use Maxwell skies and materials. Form.z immediately crashes with a render texture error warning in this case. Crashes immediately on every single model that does not have maxwell dependencies.
So here is where I start to guess on solutions. My best theory is that Maxwell, Vray and Thea do not play well together at all. In order to use one of these you must uninstall the others.
When I open any of these models on my other machine (no vray or thea) they work fine.
In the end you are correct. They are 100% incompatible.
I do realize that these programs are a work in progress. Hopefully some day soon we will have the ability to install multiple render engines into form.z the same way that other programs do.
System: Windows 7, dual 10 core Xeon 3.2 with 64 gigs Ram,
Tech: please post this wherever it makes the most sense.