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First, can one get married on the Tourist VIsa, however, does the clock start over regarding the 30 60 day rule when she returns from Mexico to revisit the U.S. I understand that one can get married on the Tourist Visa, however, I believe one must prove that was not the intent
Yes, it is fine to get married on a tourist visa. The clerk is more interested in proof if identity than the immigration status, which they do not even inquire about.
Normally, USCIS looks the other way provided that the B2 alien entered the USA legally, even if dual intent is in place. If the marriage takes place after they have been here 60 days (or 90 if possible), then the PRESUMPTION of dual intent is no longer automatic. See HERE. However, the parties want to state that they did NOT INTENT to be married until then and there, that this was a last second thing. This is because she cannot have had dual intent (to circumvent the fiance visa process) with a B2. Assuming USCIS believes her (and normally if this is done past 60 or 90 days, they so), then this should not be an issue and she can adjust to a greencard based on marriage by filing the I-130/I-485 package after marriage without even the need to go back to China.
If we were married before she left for Mexico, would we have a better chance of obtaining the necessary waivers for a spousal visa or because she left for Mexico we would have to apply for a spousal visa. OR would she need to cancel her mexico trip however, there would be no guarantee that while on a tourist visa she had a change in status would be approved.
You are making this too complicated. She is on a B2, correct? If she gets married in USA after being here for 60/90 days, then she can adjust from USA to a greencard. But if she goes abroad after marriage, she will have to adjust from abroad with an I-130. OR if she files the I-130/I-485 while here but goes abroad while the paperwork is in process, the paperwork will be voided and she'll have to start over abroad.
If you marry abroad, then you'd have to petition for her while she is abroad, and she'd need to interview at the US Consulate/Embassy in China.
The standard waiting time is about 6 months from start to finish whether she does this from WITHIN USA, or, from abroad.
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