PTFE tape is used to reduce thread friction and enable a greater clamping force.
I think it has to be worth a try in your case - fit the bb with plumbing tape and take it for a ride round the block and see if it comes loose....
Your first paragraph is incorrect. Any clamping force is between BB and flange. PTFE would have no effect. It is a sealing tape. However the second paragraph is worth a try as the tape may well fill the gaps in the worn threads and stop the thread pulling out as you tighten.
Your first paragraph is incorrect. Any clamping force is between BB and flange. PTFE would have no effect. It is a sealing tape. However the second paragraph is worth a try as the tape may well fill the gaps in the worn threads and stop the thread pulling out as you tighten.