First, the leather is pretty thick. You are also asking it to make a pretty severe bend. Did you try casing it and using a bone folder to help form the curve? You mention a thonging chisel. Those are for lacing, not stitching. You obviously can use them to create stitching holes, but it is not proper. Get a stitching iron. Lots of them discussed on the forum and Nigel Armitage has extensive reviews on his website and on Youtube.
Weldwood contact cement is what I use a lot, red can. You can buy it at Lowes if convenient. Put on both sides, let dry and then put together. Should hold well.
If you use a drill, still mark your holes with a straight line and tool such as a stitching wheel.
Finally, awl work is a skill you should develop. With practice, it becomes automatic, you don't even think about exchanging the needle and awl. I thought it was ridiculous at first, but stuck with it and now it is easy, almost robotic. If you pursue it, make sure you have a good, sharp awl. I prefer one with a short blade, 3/4" to 1" max, but YMMV.
Good luck.