Leakage in my pipes, how to densify/seal?
You should be using fittings that are applicable to the application, and should not be mixing and matching thread types. If you need to go from a fitting with tapered threads, to a fitting with straight threads. Then you should use an appropriate adapter. 

I'm not sure why you're anti-PTFE tape. It's cheap, easy to use, and works well. PTFE tape is not specifically designed to seal threads, even though that's how it's commonly thought of.  In fact, PTFE tape reduces the friction in the threaded joint. This allows you to get the joint tighter, which lets the fittings seat together properly. This is especially effective with tapered threads, though can also help seat straight thread fittings as long as it's not over applied.   When working with straight threads, you'll find that paste dopes work better than tape dopes.

If you're looking to design a system that can be disassembled, and reassembled  often. Then you're going to want to use fittings that don't rely on the threads to make the seal. The fittings may be held in place by threads, but the seal itself should be independent of the threads. If the seal is dependent on washers and/or O-rings, you'll want to be careful ow much you tighten it down.  Crushing and/or deforming the washer/O-ring, can actually cause leaks.