I have also used a feather weight interfacing that I cut into my own strips once I ran out of the scraps of it I had saved from another project.
I only used it on one side of the batting, and it held well enough to get everything in place. It is needed to keep it all from shifting while you assemble the sandwich and baste it. If you are going to do aerobics with it before layering, then I'd suggest bonding the other side too, or stitching it together either by hand, or with a large zig-zag on the machine.
The idea of slightly overlapping the edges and giving them a trim would solve the "matching" issue. Unless your curves were very gentle, the tape option would be out.
If you have tiny slivers of space where the batting doesn't meet, you will probably be OK. Large or long gaps will be more noticeable.
Don't you just LOVE it when you buy a product, especially online, and there aren't any/many directions? I still have an aftermarket darning foot waiting to be used, because I don't have directions (in the pkg or online) for installing it on the machine. It sounds like you managed OK though, and thanks for the heat tips on the product!