I've glued ipe with TB III as well and had great results. I also use a freshly sanded surface, but I use thinned TB III (good and runny, yet opaque, not watery) as a sizing coat on all components first. I get it good and wet, a solid coating, and let it sit while I prep the rest. Then right before glue up, I scrape the excess away with a putty knife or wood slat, wait just a moment for the surfaces to get tacky, but not fully dry on each piece, and run a new thick bead of thick glue down the middle. I spread that, put em together and start clamping.
Dazv: lumber is sawn lengthwise in trees, because the other way doesn't work! So, generally, boards have grain running "pretty straight", and pretty straight is good enough. HOWEVER! Think of a board milled out at an angle in a tree. Think of a straight board milled from a tree with a big crook or bend. I have made a good handful of bows with "pretty straight" grain on the belly, and they were fine, but I have had several fail because of belly grain, too, each time where it ran back to front at about 15 degrees or a tad more.
The trouble with ipe (purpleheart, massaranduba, bloodwood) is that the growth rings and grain in general are darn hard to see. There is only ONE test I know of to demonstrate whether the grain will hold.
Get your half inch thick bellies and side taper them a bit. Then bend them, without a backing until you are sure they will break. At least half the draw length. Bend it until you wince, turn your head away, and start closing your eyes. If it doesn't break, make your bow.