I got company grade article 15 during AIT and demoted to PFC. If my ERB says PFC, how am I getting my rank back once I get to my unit?

After receiving a demotion as a result of Article 15, your clock for promotion starts over. You are now treated as though you just got promoted to PFC. You will only be promoted at such a time that you have met both the Time in service and time in grade requirements to be eligible for promotion. In addition, after receiving an Article 15 you are flagged. A flag stays in place until your full punishment, or the duration of a set suspension of punishment, has been completed. To ensure that this does not stand in your way later on, make sure that you get the flag removed from your records after completing your punishment, or if a suspension of punishment was put in place make sure that when that period has been fulfilled the flag is removed.
Once your flag has been removed, promotion to DOC may be automatic after accumulating the required time or you may be eligible for waiver. Waiver is subject to the approval of your unit's command.

One thing immediately comes to mind when these types of questions are asked of this forum.
Did you talk to your chain of command?
After I received my first, of several, Article 15's, I talked with my squad leader, assistant squad leader, and PSG about what I could expect next. That's who you should be asking. There's a lot of information you have not provided, that precludes anyone answering your specific question. We don't know the circumstances of the offense, you say others were doing the same thing, I think, and got away with it.

We can give general advice, and SGT Gunderson gave you some good information. But that's all it was was information, not something that you could say "Hey, SGT Gunderson said this is how it's supposed to be so why is something else happening?"
If you want that type of advise I suggest you get an appointment with the JAG.
You also seem to be concerned about the monetary consequences after your Article 15. Maybe you should have thought about that before hand.
There is a lot more I could get into here, but my very best advice to you is this. Talk to your chain of command.

I have never seen someone get their rank back in the way you're asking about. I believe it can be done at 18 months TIS w/ waiver. Also, I think the commander can promote you immediately if he sees fit, but in all likelihood he won't. You got a company grade, so you lucked out. Company grades don't follow you to your next unit, but you will be treated as a PFC. I would advise not broadcasting your article 15 as it might put you on people's radar as a dirtbag. Work your ass off and earn the waiver. You'll make SPC sooner than you'd think. SPC is a bigger deal than you might think. Often SPC are required to fulfill the role of a teamleader. Your words and certainly your actions suggest that you might need to mature a little bit before you're ready for the responsibility. Spend your time as a PFC developing your skills, because there's a possibility that you could be called upon to lead once you make SPC.