thank everyone for the advice! I'm still in a quandary but somewhat forwarder. This project is borderline too-much-work-just-give-it-up, so I need to be sure it's going to be perfect (well, as close as my skills allow), or I just won't feel motivated, and I don't mind perhaps going to excess to make sure.
Charlotte, this batting had probably been in my stash for a while, well back before 2015 and any receipts are long gone. I suspect I would have got it at joanns.
Tartan, doubling the 2oz one is an idea I didn't think of. I got a sample ready to test, but I think it's too bulky.
Wesing, the sections are about 6 blocks in size, and the original section is in the middle not a corner... it can't really hide. But being surrounded on three sides and with zig-zag joins (it's in hexagons) it might pass. Though I prefer the softness of the original...
Sewingpup, I looked at sample cards, but most of the samples would not be thin poly so it seems like too much when I could just order one more batt :-) . Side note: it's amazing how things have changed... seems like it used to be that everyone just used poly.
In the meantime, the calipers I ordered have arrived... and a quick test, over 8 layers, it looks like the low-loft is thicker than the original! the soft and bright and original are similar. But I need to do a more careful test. Am I mistaking thickness for stiffness? I would guess that some stiffness relaxes with time and manipulation?
I am going to do an actual quilting test. I've cut 6" squares of the same fabric for top and bottom (the quilt is scrappy front and back, so while I don't think that's the thickness/stiffness problem, I can still hope!). I'll do a little bit of quilting comparing battings.
And my finger hovers over buy now button for mountain mist quilt-light... if I just try one more, maybe that will be exactly perfect...