How should I apply for a U.S. visa with an approved I-140 for NIW?

If you did not enter legally, you may be triggering an unlawful presence bar when you leave, so be careful. To get a waiver of the unlawful presence bar, you may need an anchor relative with US citizenship or permanent residence status and may need to show extreme hardship to that person.

You need a consultation appointment with an immigration attorney.

You should have a comprehensive consultation with an experienced immigration attorney before deciding to depart the US. See if you may be eligible for 245(i). If you were to depart and had accrued unlawful presence, 3 or 10 year bar would apply that could be waived if you qualify.

You need a consult. If you return home and have accrued unlawful presence, you will incur a 3 or 10 year bar depending on the amount of time you overstayed. This can be waived with a qualifying waiver. You should also see if you are eligible for Sec. 245(i).

If you have been in the country illegally for more than 1 year, when you depart, the Embassy/Consulate will deny the visa since the punishment for being here illegally is 10 years. Unless you are married to a U.S. citizen or resident, or have parents who are legal residents or citizens - one of them would file a waiver with USCIS. However, it takes 2-3 years for a waiver to be adjudicated. Do not leave.

You may need a waiver to return to the U.S. You should ask your lawyer what to do, or meet with an attorney.

You need to retain experienced counsel for this. There are a number of eligibility and timing issues at play.