Checkin through the thread for possible solution for your strange situation I found this:
https://xdaforums.com/nexus-7/development/rom-slim6-alpha-0-6-unofficial-t3425537/post68519717
Is there still super user inbuilt? In case it is just activate it and flash TWRP by the App (on Playstore), this would be super easy but I seriously doubt you missed this option all the time while looking for solutions...
Edit: what still is something which needs explaining is how can the recovery revert to stock in first place? You didn't even flash the factory image but only a custom ROM, right? This absolutely shouldn't happen and may point to further, more severe issues, like something about the hardware which could also be related to your issue with not being able to establish a connection to a computer, pure speculation but what you described is hyper weird...
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