I don't think it's a problem either of TWRP or SU. I have tried many versions of both (including the one you proposed) and I had always the same problem. Strangely, I managed to fix this issue late last night in this way:
While I had flashed TWRP and rooted with SU 2.78, having the problem with the continuous app force closing, I decided to flash stock recovery with flash tool once more so I can restore through this stock recovery, a complete backup of 7.0 (NON rooted) that I had taken 2-3 days ago. I did that, and although I can't explain this, I am with android 7.0 now, with *stock* recovery, with root that works ok (I have to say again that the backup was NON rooted!), but without SuperSU installed or in my apps drawer! And everything seem to work ok.
Do you have any explanation of this?