The edge where this crochet chain happens is just going to be seamed together with a crochet slip stitch at finishing anyway so I don’t think it matters whether you use a crochet chain or just cast on the additional stitches. Bare in mind not all pattern designers make use of a knitted cast on, if they know long tail and backwards e loop cast on which is not ideal for a long section, then they may turn to a crochet chain to get a longer set of new stitches.
From what I can see it happens on right back and left front. I would just get to the point where its needed and use a knitted cast on, working it a little loose in tension.
Or you could work the crochet chain, and pick up a new stitch in each chain loop. There’s nothing about removing it later, it’s just to provide stitches.
Or you could make it with scrap yarn as a provisional cast on and later, when it comes to finishing you would need to remove it stitch by stitch and work the crochet slip stitch seam into each of the stitches that the provisional was holding.
Personally I’d rather not be trying to undo a provisional and crocheting into it at the same time, that’s me, I’d rather knit cast on. You are anyway going to turn to work the wrong side row so turn, knit cast on 35, then begin the ws row.
I’ve used a crochet slip stitch seam and it’s very nice, more stretch, flexibility and softness than mattress stitch, but it often needs some experimentation with crochet hook size and how much to tighten each stitch so that the result is nice. I undid mine a few times and redid it until I was happy with it, that’s why I wouldn’t want to be working it into a set of stitches with a provisional cast on yarn, I like the safety of knowing my fabric and stitches aren’t going anywhere.
You could choose to mattress stitch the seam instead which would be fine, in which case you would have no need for a provisional cast on.
Or you could graft or faux graft that bottom section of the seam to make the back and front blend at the join, it would be a bind off to a provisional cast on (you could skip the bind off in the relevant section of stitches and fully graft), but I am not sure the rest of the seam would then align smoothly, one part being flatter with the graft and the other part being a stitch in from the edge with the crochet seam or mattress stitch seam.
Matress stitch could be worked tightly as usual for the cuff down to this straight section and then worked loosely as a faux graft, bind off edge into cast on edge across those cable stitches to make a neat seam. I’m not convinced it’s needed though.
I think there are enough options for finishing the cardigan without worrying about this 35 stitch crochet chain. It would be a shame to give up or be unable to make progress just for this one part.
What do you think?