My custom headers are one piece, but my hooker adjustables are a total of eight pieces. The coating is absolutely no problem during assembly. At least, not on mine.
I don't use anything to seal the tubes to the collectors, and even though they were intended to be race headers, I added flanges so I could plumb an exhaust system. I have no leaks even with mufflers and 30 years of use.
As for who... I have no recommendations. The company that did mine was an off road shop in riverside CA. Very reasonable I thought. Maybe that's why they are closed.
Install of the hookers is the easiest under chassis headers I've ever done. Center two tubes up from the bottom, front tube in from the top, rear tube in thru the hole in the inner fender, slip on and bolt up collector, done. Maybe ten minutes per side. The only one easier is my fenderwell header car, no inner fenders at all!