The law reads "30 days" Any day or partial day in the US is considered a full day under the law.
The CBP especially at a port of entry from a contiguous country are particularly negative about allowing someone back in who has just completed any nonimmigrant stay, not just a J.
Initially the way you wrote your question it seems as though you were going to Mexico for a wedding Then to re-enter the US after going there for a wedding to collect your belongings and leave makes sense.
But the CBP sees this as you trying to manipulate the law to stay AFTER AN ALLOWABLE grace period to prepare for travel.
They just don't like it and have discretion to refuse you entry.
If you are not subject to the 212e 2 year home residency there was another option for you to have filed for a change of status to B2 to be allowed to stay for the wedding. You cannot do that during the grace period.
I would put your risk level at medium to high. This is summer season, the officers are working longer shifts and handling twice as many applicants for entry, they are hot, tired and grouchy. You are seeing a discretionary entry very close to after the end of your program.
If your program end date was July 1st your grace period started July 2nd so you could depart on July 31st and not be an overstay.
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