With regards to ST's depression, or supposed depression, IMO, I don't think it was a ruse I think it was a beneficial byproduct of preparing to disappear, and learning how to make explosive devices.
If we think about it, he would have done these things alone or with only select individuals. This could be misinterpreted by family and friends and even roommates as wanting space, and distancing, to be alone, etc. those things could be seen as onset of depression. Then he most likely went along with what they thought was going on so he wouldn't have to explain why he would spend time alone or whatever. I hope this makes sense. Jmo...