I made one of these once, not the coat, but the shorter jacket, in the largest child size. You were planning from the beginning to make the coat more than one color? Is that the case? If so maybe it would look all right to make a button band in the second color. You would make the lower part as big around as you can minus enough for decent sized buttonbands. Then when you pick up the top part you would pick up what is there and when you have it done pick up stitches along the whole front edge on each side and knit on button/buttonhole bands.
If you were planning to make the coat, maybe you could start over and make it shorter, like a jacket or a shorter coat.
The losing weight option never really works for me, but is a great idea if it is realistic. With this option or the button bands you would pick up fewer stitches than it says to. I can't remember what kind of system they gave for picking up the stitches along the top of the bottom part (for instance, pick up one at the end of each garter rib, or whatever), but I would do it the way they say only over fewer stitches.
I've heard dying acrylic is harder than wool and a blend might not dye well. I don't have any experience with dying, if you've done it you know more than I do about that.
If you can find the yarn on e-bay there is a chance you could find the same dye lot. I have found yarn from different sources with the same dye lot. I was amazed, but happy. If you find the yarn, ask about the dye lot.