How can I fix my supplemental petition to add my wife?

Depending on the timing and where you were married, it may be simpler to add her at the DS-260 stage/consulate processing (assuming you are overseas).

You should wait until the I-526 is approved, then at the immigrant visa processing stage (or if you are processing in the U.S. when filing for adjustment of status), add your spouse.

Advisably, work with an EB-5 attorney to ensure the right response is forwarded to the USCIS.

This is not the process to use. You do not need to refile an I-526. It will require a separate fee. One option is to interfile information for your wife in the pending I-526, but interfiling does not work consistently, as the mailroom staff may misplace your documents and they will never be added to the pending case. Best option is to wait until your pending I-526 is adjudicated and when the case is approved, it will be sent to NVC, at which stage you will send a copy of the marriage certificate to NVC and ask NVC to add a derivative spouse to your case and issue a visa bill. Ensure you work with an attorney to help you with this process.

I am unsure that an entirely new petition would have been necessary. Perhaps an interfiling would have sufficed?

Given the fact that you want to add your wife as your dependent and the USCIS has not yet adjudicated your EB-5 petition on Form I-526, there is still time to add your wife. Once your petition is approved, if you applying abroad, your approved petition forwarded to the National Visa Center. When you provide documents to the National Visa Center as well as when you prepare and file your immigrant visa application, you can then add your dependent spouse.

You do not need to refile the I-526. You can interfile a copy of your marriage certificate. However the process of adding a spouse is done after the petition is adjudicated when the file goes to the National Visa Center. If you in the U.S. and plan to adjust, just file with her, if she is in status.

You do not need to add your wife now. Simply add her at the NVC or AOS stage. Prepare the marriage certificate as well as her personal documents in order to secure a CPR status for her.

Do not try to do this yourself. Immediately retain a qualified investment immigration attorney.

Wait until the I-526 is approved and then add her either by filing her own I-485 if you are lawfully in the U.S or send the marriage certificate to the National Visa Center and they will add her to the fee bill.

You do not need to file a new petition. When your EB-5 I-529 is approved, they will either have you file an I-485 adjustment of status application or send the case to the National Visa Center for final green card processing abroad. In either situation, you can add your wife.

As your immigration lawyer could probably tell you, you do not need to amend your petition to add your wife. When the I-526 is approved, you may simply add her to the immigrant visa application or adjustment of status application with your marriage certificate to prove the marriage relationship. Filing a new I-526 would require the new filing fee of $3,675, which is not necessary. Doing an interfiling does not always work because the mailroom staff that opens the petition and sends forward for processing does not always know what an interfiling is and duplicative petition will just lead to confusion and delay of adjudication, which also is not necessary.