your B1 should be multiple

there is absolutely no risk for you at all if you go to mexico. There is a big chance that they won't renew the time you are allowed to stay because your i-94 is still valid. You won't have to turn in your I-94 at all since you are only going to mexico for a short time.

If your spoken English is good, then maybe you will stand a better chance if you can provide them with a good reason why you have to stay 5 months.