This is incorrect: you don't need to be in the US at all to apply for any immigrant visa. I believe the Indian body shops perpetuate the fictitious idea that, similarly to naturalization, there is also a required time to be present in the country before being eligible for a GC. While I can see how it fits into their business model, it could not be further from the reality. Not only there is no required time to be in the country but there is no requirement to be in the country at all in order to obtain an immigrant visa - it can be all done in the home country and you can enter the US as an LPR without ever setting foot there before.
This is also incorrect. You can adjust into immigration status from any legal status in general, there might be different complications depending on status (e.g. you might not be able to leave and re-enter the country in some or you need to follow 30/60/90 rule with others) but there is no requirement to be in a dual intent status for AOS. If worse comes to worst you can always do consular processing and only leave the US to stamp your immigration visa in a foreign consulate and re-enter in the new immigrant status.
The "dual intent" concept is for the non-immigrant visa. If you don't want to read the official explanation, posted around here many times already, it means that you don't have to prove strong ties with your country when applying for such a visa. For visas without dual intent you have to prove the lack of immigration intent. For dual intent ones you don't.
You are completely right and I now realize that I worded my comment poorly. You are right in that you don't have to have a dual intent visa to apply, but take my case as an example. I'm in the US on a non-immigrant work visa, but I haven't applied for EB2-NIW because I know that if I get denied, then I would have proved intention to immigrate and getting another non-immigrant visa will be close to impossible.
I wouldn't have that problem if I was on, say, an H1B Visa.
Well, if your immigration petition had been denied while on H1B you wouldn't be able to extend it past 6 years and for most H1Bs, whose bodyshops file for GC at the latest date possible, it would mean going home right away or, at best, at the end of their last one year extension. I am not a lawyer and might be overlooking something obvious, but I can't imagine a situation where going through a non-immigrant visa (even such as H1B or any other dual intent) and AOS had been somehow better than straight immigrant visa if you intended to immigrate and have qualifications for a non-sponsored GC.
