Hi Erica, I'm an RN and med student.
1. RN work (i.e. clinical exposure) is highly valued. Do volunteer work only if you like it. Certain competitive schools like to also see some research.
2. I did not do a formal post-bacc (too expensive), I just went to the local university and took the classes I needed while I worked. The trick is letters of recommendation. Try to get some lined up now while you are still in school.
3. The single mom thing will probably be the biggest challenge for you. I'm not a single mom, but this is probably universal: med school and residency will take a LOT of time away from your kid. You need backup childcare, and backup to the backup. Hopefully you can get into med school near your home and hopefully you have relatives who can watch your kid when you're on call overnight, etc. I've never met someone who went through med school as a single parent, but there are people on mommd who have done it like Kathie, DO. Good luck to you. I know it's a hard decision.