I'll offer some ideas along with those of others, based on my time with airbrushes and hobby painting. When I started 50+ years ago, I had such poor results and so many problems that I thought airbrushing was just not something doable for me. Clogging, spattering, inconsistent paint condition, etc.
But seeing that others could do it well, I stuck with it and in time I learned some things that just cannot be overlooked. If we don't take the proper care and preparation everytime, the gremlins will surface. The following are just my suggestions.
When getting ready to remove any paint from the bottle, first ensure a thorough mixing of the contents. I use either plastic chopsticks or small metal spatulas to ensure all of the settled solid contents are lifted from the bottom, shaking the bottle just doesn't work well for me. Then the Badger mixing tool, or similar, churns everything up to a uniform blended consistency. If any thick chunks are allowed to be in the airbrush, clogging will immediately be the result.
Same with adding thinners, it's important to ensure a complete blending with the paint before adding to the airbrush. With regard to getting the paint out of the bottle, I recommend using a pipette or glass tube eyedropper to draw it out, as opposed to pouring over the side of the bottle. It's really important to keep the bottle tops and threads clean, or they won't seal properly for storing the paint.
I struggled with caring for my airbrushes, not really understanding how the components work together for proper function. Finally I sat at the bench and disassembled, then reassembled repeatedly, many times, until eventually I understood how to properly clean, lubricate, assemble, seat the parts and return the airbrush to service.
As the parts are small, nothing really needs much muscle when assembling, especially the tiny front end bits like the one that broke on your Iwata. Only finger tighten, then just a tiny nudge with the included wrench is plenty. Once you get the frustrations out of your way, airbrush painting will bring you years of enjoyment.
With regard to air pressure, I'm usually at 12-18 psi, but in some cases as high as 20-25. It all depends on what psi works best, testing the results will reveal what to set it at, before painting the subject, for each session.
I hope everything goes smoothly for you, as you proceed.
Patrick