You haven't set the new background file to "Multiply" blend mode.
Can you post the new background file by itself, so I can see what it looks like? I think you might be going about this the wrong way.
This is a very good idea. The more you can do this, the better and faster your work will be.
Have you ever made an action before? I wrote a simple tutorial here.
Also, some general info here.
And of course, this one will slice heaps of time off your output workflow.
2) When it comes down to technical basics, the files you're sending to the lab are just a bunch of ones and zeros. Simple binary data that comprises all digital files. The screen upon which you edited is completely irrelevant. All that matters is for your calibrated screen to show those ones and zeros the same as the lab printed them.
1) For the purposes of calibration, you must ignore those profiles. They'll become important later, but can only be utilised after you are confident with the calibration.
I have to leave now, for a little while.
Basically, I think your settings should be 300/300 (rather than 300/450) and High, or even Medium, not Maximum for the image quality.