After you get the first side if the tire on the rim and have the tube installed with it's valve stem in the hole of the rim, then inflate the tube just barely enough to almost round it out. Then it might keep the tire from being too floppy and hold it's first bead somewhat centered all around on the rim. But at some point while getting the second sides bead on the rim, you might need to let all the air out.

I've had similar issues even with 23 and 25 mm wide folding tires that are brand new. They just seem like they aren't going to fit properly. But they do, and after they get inflated to their normal operating pressure and are ridden for a time, they seem to take their proper form for the next time you have to remove them to change the tube.