@hummlbach Those are all good, difficult and complex questions indeed. These are also questions that are beyond my competence and insight, as I know little about the manufacturing process. I'll do my best to give you a broad overview of how the Pinephone is being born and hope that you will be able to deduce answers from what I write.
Large, complex and custom built components - such as for instance a LCD + TP panel - are very expensive. Custom plastic/metal molds are expensive too, as is extensive retooling of plastic/metal molds. To maintain the Pinephone price-point, some of the components - such as parts of the case and the LCD+TP - have been sourced from reliable vendors with two major factors in mind: quality of the components (+ their long-term supply safety) and the ability of the component in question to work "seamlessly" with mainline Linux. Then there are the many custom components - including the main PCB, the inside layout of the phone, parts of case, etc., - that are assigned to design studios and manufactured by factories the studios work with.
We do not have the means or ability to vet every vendor (the number of reasons why is beyond the scope of this post, but spans from cultural to practial) nor can we influence which factories design studios work with. Now, with that said, all component vendors meet all standard manufacturing practices that apply to 99% of quality electronics on the market. We cut no corners and work with reliable partners only.