Hi Sweetpea and welcome
I can totally understand your anxiety about another pregnancy - especially since you had suspected IC and hyperemesis - poor you

We have all been there, and it took me 4yrs to feel brave enough to take the plunge again after Evie.
The fact that they're agreeing already to a stitch is a huge plus, and you should feel fairly confident that this time you'll make it to term. Last time an emergency stitch worked for you which is reassuring and indicative that your cervix is probably weak, but not hugely incompetent.
I doubt that dehydration contributed to the IC hun. It can cause ptl, but not 'silent' dilation of the cervix under pressure. It might have induced early labour at 34/35wks and is what caused you to deliver early, but it wouldn't have made your cervix weak.
It is possible however that the constant vomiting placed additional strain on your cervix at 20ish wks, but again doubtful that this alone would have affected a strong, closed cervix. It might be that there is weakness there, complicated by the sickness - much like a weak cervix is put an extra strain when supporting two babies, when it otherwise might have held up under the pressure of one just fine.
It's impossible to say for certain what causes IC, or in fact if it will recur, but I personally wouldn't take any chances and if the stitch is offered, go for it. I'd be fairly confident that you can carry to term with a stitch. It sounds as though you did go into early labour in the third tri and that was probably unrelated to your cervical issues. You had already begun to contract before stitch removal, and once removed your cervix was able to dilate fully under the influence of further contractions. This might have just been one of those things which may not complicate any future pregnancies.
Jees I've just waffled - sorry lol. Good luck xxx