1. Based on the medical records from the treatment taken here in the U.S. and attending physician statement, the claims department of the insurance company determines whether something is a pre-existing condition or not.
2. Please look at #1. Determination is an after-fact based on all the medical records submitted. Therefore, based on something initial like that, it may not be possible to tell whether something is covered or not, whether it is an acute onset of pre-existing condition or not. It can only be told for sure when the claim is processed, that is in most cases after the treatment.
3. I don't suggest that. When you buy two insurance plans, it does not work like the way you think. Both plans are effective at the same time, considered secondary to any other plan you have and therefore, both companies would have to do coordination of benefits and would only pay a portion of the bills. And when two companies are in two different countries, you are looking at much bigger headache than you can imagine.
It is always better to buy the insurance from the U.S. based companies only. Please read
http://www.insubuy.com/guide/visitors-insurance-usa-india-comparison.jsp to understand the difference between buying from the U.S. vs India. Most of the plans in India are only fixed coverage plans for persons above the age of 55 years. Read
http://www.insubuy.com/visitors-insurance/ to understand the difference between fixed coverage and comprehensive coverage. Also, read people's experiences in this section and on this discussion forum before deciding to buy from India.
In your case, you can balance the cost vs benefit by choosing a deductible of $250, $500 or $1,000. $100 deductible is too low and $2,500 is too high and you are considering only the extremes.
I suggest that you buy one insurance from here in the U.S. such as Atlas America and only one.
I hope that answers your questions.