Huh. I got a batch of Antron fleece from Georgia Stage earlier this year, and I've had no problem at all dyeing it. My Skeeter came out a lovely vibrant orange, and I have a few feet of gray for an upcoming project. It makes me wonder, are some sellers claiming to sell Antron fleece when it's actually another brand?
As far as hiding seams goes, yeah, mine don't "disappear." If I pick the seam--which I have to do several times, with the light hitting the seam from several directions--then I get a kind of mohawk that I have to brush and trim down. I've been experimenting with the earlier-mentioned toothbrush method. The one I'm working on is all fur except for the eyelids, but I'll see how toothbrushing works out on the next puppet.