@Lachica crudely drawn, the thread path I mean, is almost like two opposing runs of backstitch

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If I run that down the back just towards the point, it should take a little of the tension off the fabric at the front.

After fiddling around with it, I probably also need to pleat below the point slightly, which will bring the two seam allowances together. With a 5mm seam allowance either side of the cut, that leaves 10mm of material width bunched up under the infill piece, which is probably not helping matters. If I pull that bunch together into a pleat immediately below the apex, then do that curlicuetype stitch above, hopefully that will fix it up nicely.

As for covering it, more often than not, I tuck shirts and tees in. That said, the belt will cover the waistband, so there'll only be the point visible. As you say, probably not even noticeable... but if I can bull it up with a few extra stitches, it's worth an extra couple of minutes of faff.

And, yes, stitch all of the width of the stitch through both layers for nice even stitches. To trim, fold the hem right back on itself, right sides together, so only the surplus that you want to trim off is poking up.

Thank you all very much for the tips. I do have a twin needle, but have never used it, so I’d be wary of using it on my first attempt. I’ll have a go with the zigzag type stitches and see how I get on.

Very neat @TangledBloke . I don't know what the proper name is but I call it ladder stitch, I use it to close small gaps without thread showing on the RS. However, presumably you'll wear a top which covers the insert, so as long as the stitching is strong those tiny visible stitches will do the job fine. Anyone close enough to see them probably shouldn't be there, at least in public.

On Jane I use either wht your manual describes as multiple zigzag, stitch 8 or for daytime t-shirts, I like the effect of your stitch 20, which mimics a coverstitch quite nicely.