I’m not a lover of twin needle work on  jersey.  Never seemed to get a good result. 
I prefer a 3 step zig zag to a normal zig zag.  It’s wide enough to make the hem lie flat and doesn’t give the problems that a plain one often does.  That’s a nice finish @Ouryve.  Not used that before. 
I used to use a feather stitch and also a Greek key stitch.  Either of those works because they go back and forth building in elasticity.  But they take time. 
Nowadays I have a cover stitch machine.  I sometimes like to sew it upside down.  It’s a nice look but is also so easy to get right to the edge of the hem!

Or a jersey needle. My janome prefers jersey needles, pfaff prefers stretch. Or it might be the other way round. Skipped stitches if I get it wrong so I soon find out.

@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.