It’s not what you want to hear but……..I would want to sample to make sure I got the results I was hoping for. Have you done a ruler wrap to establish your sett?
You have quite a significant size difference in the two yarns. I tried to figure out what the yards per pound were in the yarns you are dealing with. My best guess, without seeing the yarns but with the information you have given us is - your warp must be about 500 yards per lb. and your weft about 1/4 the size at 1818 per lb. I'm thinking that almost anything you use will be lost in the size of the warp threads you are using. As I said - I'm totally guessing but you have the yarns handy so you can tell if I'm close at all with my results ;-) Do you have enough of the Matchmaker Merino to use as warp? I think you would be much happier with the result. The loom waste can be used for your fringe, so I wouldn't be as worried about waste because you are weaving a scarf.