How do you sew a drawstring channel through this many layers without the machine freaking out?

Four plus layers of canvas is a lot, but a basic machine can usally get through it if you set it up right. A denim needle alone won't fix it. Go up to a 100/16 topstitch needle, since it has a bigger eye and the thread glides through instead of shredding. Lengthen your stitch to around 3mm or a bit more. Short stitches in thick fabric just mean the feed dogs have to move the whole stack twice as often, and that's when you get skipped stitches and loopy bobbins. Yes, hammer the seam. Seriously. I keep a rubber mallet in my sewing kit for exactly this. Lay the seam allowances open over a tailor's ham or a piece of scrap wood and whack it a few times. Canvas flattens down surprisingly well and suddenly your stack is less of a cliff. Then clip or grade the seam allowances underneath the casing so there aren't lumpy intersections. And baste it, yes. Hand baste the whole channel width before you sew it. On something this thick, pins don't hold layers togehter, they just make bumps.

The loopy stitching usually means the presser foot is riding up on the hump at the seam edges and losing presser on the fabric. Slide a folded index card or an old thin ruler under the back of the foot when you hit the thick spot, so the foot stays level. Some machines let you drop the presser foot pressure a notch too, wich helps a lot on canvas. A walking foot is worth trying if you have one, even though people mostly think of them for knits.

For the buttonholes, the same fix applies. Hammer the placket flat first, and put a piece of wash-away stabilizer or even tissue under the area. But the real trick: start the buttonhole after you've crossed the thickest seam, or position them so the foot sits on level ground the whole time. If the foot can't climb, the sensor thinks it's done and stops short, which is exactly the half-length jam you're describing. Some buttonhole feet can be slid manually on non-automatic machines, so if yours is the one-step sensor type, that coud be the whole problem.

If none of that works, hand-worked buttonholes with a buttonhole stitch are honestly not bad on a placket, just slow. Or a tailor with a proper machine will charge you very little for six buttonholes. Not a failure, just tools.

I have to disagree on the walking foot bit. I tried one on a thick canvas tote project and it made things worse, not better. The walking foot sits taller than a regular foot, so on a hump it loses contact even sooner and the layers go wandering. A plain foot with something wedged under the back of it did a better job on my machine. Maybe it varies by brand, but I would not send someone straight to a walking foot for this. What actualy saved my project was cranking the wheel slowly by hand through the chunky spots instead of using the pedal. Full control, no skipped stitches.

Before you blame the machine, run a proper diagnostic: cut two scraps of your canvas, stack them to four layers, and sew a straight seam with a brand new needle. If it skips there too, the machine itself may have a worn hook or a timing issue, and no amount of wedging will fix that. If it sews cleanly on the test piece, then it is purely a feeding problem and the wedge-under-the-foot trick from earlier should carry you through. Also check what thread you are using, because a heavier topstitch thread run through a small eye can produce loopy bobbin threads that look like a tension fault when they are relly just the thread fighting the needle.

One thing that helped me on a similar project was changing the assembly order. I sewed the casing channel flat before joining the side seams, so I wasn't stitching over seam intersections at the corners. That's where my machine really choked, not the channel itself. Also try loosening your top tension a hair for the thick passes - a loopy bobbin isn't always the bobbin's fault, tight top thread can pull loops to the back too. And honestly, don't stress the inside lumps on a kid's jacket. It gets worn over a hoodie and dragged through mud. They'll outgrow it long before they inspect the seams.