I always enjoy these draw reins posts! Everything we put on our horses is a gadget. If you aren’t a good rider less is better for your horse and if you are a good/experienced rider you are going to understand how to use the various tools we have. For example: I never use a martingale, running or standing, because my jumper doesn’t need one. I don’t even own one. I have used draw reins with him at various times and they have been very helpful. But, I know how to use them.
OP: the best answer probably is to have a trainer or experienced rider show you how to effectively longe. If you already know how to do that, then ask that person if side reins would be useful for your mare. And then ask them, if they say “yes,” how to use them properly and if they can help you with teaching her how to longe in them.