I don't see a problem with a pillow UW - if it's basically empty at the surface it will just get even smaller at depth then get no bigger than it started on ascent. But it sounds like a nuisance to deal with, and not likely to help your swimming speed any. Sticking a SMB under your neck is probably even less streamlined to swim with like that.
Is there a specific reason you're swimming a distance on your back? Some people like to swim that way, but if you're uncomfortable, why do it? Personally I find swimming on my back uncomfortable, plus I can't see where I'm going, and have the sun in my eyes and burning my face, and I don't think a pillow would fix any of that. I'd much rather swim forward with a snorkel (or usually my reg, as I've always got air left.) Long surface swims are exactly what snorkels are for. (I do think stowing it someplace other than your mask during the dive is a good thing to consider though.)
While there's reasons you might surface swim back to a boat occasionally, it seems unusual that you find yourself having to do long swims a lot of the time. If you're having trouble navigating back to the boat UW, it's just something you can work on and learn and this will pass. If you have to, you can always surface to see where you're going then drop back under shallow where swimming with scuba is usually easier - I think pretty much everyone does this once in awhile. If the problem is that you're often coming up far from the boat because you don't have the air to make it back UW, that's a real problem that will bite you sooner or later and you do need to work on turning your dive soon enough.