Instead of mounting the core and trying to get the piping to work, do it the opposite way. Completely unbolt the core, and hook up all the pipes. After you have the pipes positioned correctly and connected, they will support the core for the most part. Then, see where the core sits, and decide if the pipes need to be modified to adjust the core's position. If the core is close enough, then fabricate a bracket to line up with the existing bracket on the core, and a mounting point on the car.
As far as the couplers go, just make sure you used the pipe OD, and the coupler ID when buying replacements. I've used these guys in the past, and they have good quality products.
click
I'm not sure why you don't just send the whole thing back if you are having so many issues. If you are willing to keep the item, then you are accepting some responsibility for its condition and missing items. Judging by the lack of cooperation between you and the seller up to this point, you're likely to be into this for way more than the kit is even worth. Just seems like more hassle than anything IMO.