1) I want to apply from outside Canada for her. Can we apply for her from US (while she is a visitor) or she can only apply for it from India?
If she is admitted to the US on a visa that is good for a year or more, she can apply through the US. She can also apply through India if she wants. If the visa she enters the US on is good for less than a year, she will have to apply through India.
She does not need to be in India to apply through the Indian visa office - she can stay with you in the US while her visa is being processed in India.
She does not need to have been in the US for a year before she applies - her visa just has to be good for a year or more.
2) If we apply from the US which region specific requirements/rules apply to her: India or US?
The US. You always follow the region specific requirements for the region you are applying from. If there are some requirements you can't fulfill because of differences in documents between countries, attach a letter explaining why anything is missing.
In my opinion, even though it is not required, it is better to include anything the Indian checklist requires, in addition to what the US checklist requires - just in case the visa officer decides to send the application to India after all, which they can do.
3) If there is an interview, will it be conducted in US or does she have to go back to India?
The interview will be in the US if you apply through the US, and in India if you apply through India.
The visa office in New Delhi is faster, on average, than the one in Buffalo. But the New Delhi office asks for the passport very early in processing, and you are not supposed to send Indian passports through the mail. So your wife might have to go back to India to submit her passport, and wait there if you choose India as the processing office.