the easiest way to do so is to become employed by an American company and have them sponsor your permanent residency request.
That is by far not the easiest way. How many employers do you think want to play with this mess? Very few! The process takes up to 5 years just to get a green card, depending of course how fast your employer plans to act. Employers take their sweeeeet time with this process, due to legal fees, and keeping you as an employee at cheaper rate longer. After you get a green card, they'll make you sign a contract for 5 years that you'll stay with the company (during which time they give you no pay raises, and if you leave, you have a hefty $sum to pay back).
You must be a permanent resident for minimum 5 years to apply for citizenship. So taking this route, you are looking at approx 10yrs.
If you are Indian, your chances of an employer following through with this are 1% at best. So many temp outsourcing companies in US/India working together, that they dont need to go through the process to have an Indian.
But by being illegal and overstaying their visit, they won't be able to come back to the US for 10 years.
Not entirely true. The amount of years for the ban depends on how long you have overstayed. Some short overstays are forgiven, some 3 yrs, some 5.
If the visa is expired, then when he does get married, the INS will come back to him saying his visa expired and they will say he could get deported unless he can prove that it would have a damaging effect on others, in this case the wife.
Very not true. Marrying a USC forgives ALL overstays. You will not be deported. You have to prove to USCIS that the marriage is true & genuine, and based on love, not the ability to gain legal residency. The part you mention "damaging effect on others" is really used for other cases, not marriage to USC.
He can stay in the US when papers are filed. And he should, because he might not be able to return to the US if he leaves. Even if his visa is good for 10 years, I've heard cases when people were turned back at the border for something.
You can, and should remain in the US until you are granted "advanced parole" document. This document gives you the right to re-enter the US during the time your AOS application is being evaluated. If you have previous illegal overstays, you CAN NOT LEAVE until you have a greencard in hand. Leaving earlier will destroy the whole process.
so his plans are to find someone to marry, and then later be with me. which is one hell of a screwed up situation, i know. That, or he was interested in buying a green card if that is at all possible.
Fraud if I ever heard of it. Do you think he's being even honest with you? Chances are, he's giving you this line to get you to feel bad, and marry him. He hasnt worked all this time on charming you for no green card. He'll get it, and take off... while you have the legal liability for him for the next 10yrs.