You need someone with hardware prototyping skills. You can either get an unpaid cofounder, or pay someone to do it (and worry about getting a hardware cofounder later). For either of those options, you'll need someone local. Look in local hackerspaces (https://wiki.hackerspaces.org/List_of_Hacker...
It varies and it's very very specific to what you want to develop. The concrete design of your circuit matters. Also prototype building costs are usually a factor 10-100 higher than series. If you already have your prototype then you can shop around various manufacturing companies. To do that, yo...
Quality control is always a concern when working with an overseas vendor, and I can only imagine your frustration with the samples you have received. My advice having sourced from China and many other countries is that you need a person on the ground to make sure your expectations and specs are b...