In what status am I after receiving a letter mentioning they have withdrawn my B2?

Your question requires an attorney consultation. It is not a simple question that can be answered on this type of forum. There are many factors that would need to be considered and evaluated. I strongly suggest that you contact an experienced immigration attorney for a face-to-face consultation and give him/her all of the facts surrounding your situation. He/she would then be in a better position to analyze your case and advise you of your options.

I think the best approach here is to hire an attorney to see where you stand. Kind of an odd situation in which you probably should not have been approved for the H-4 if the request to obtain it was made after the expiration of your I-94. Not that it is impossible, but in order for you to have been approved of a change of status to the H-4 some recognition of the fact that you had violated the prior status should have been acknowledged and I am not sure that this happened. This is one of those problems where you could do lots of work and never get anywhere for lack of finding someone over at CIS who will look into it and fix it. But if it was a premium processing case, you may have access to the folks at that unit to figure things out. And you might want to speak with the attorney who helped you and see what they did or didn't do. Wish I could give you a better and mor straight forward answer, but it ain't all that easy. Find someone who can look into to it for you and make sure that all is square. Good luck.

If your husband timely applied for H-4 derivative change of status for you and it was approved (and you have now withdrawn your B-2 extension request) you are now considered to be maintaining status as an H-4 through the date of expiration cited on your most recent I-94 (which should have been provided at the bottom of the I-797 Approval Notice for your H-4.