+1 to everything hahahut said. Here are my thoughts on your questions:
1) I don't think this sentence works " Inreturn, I will give your program as many years as is takes of uncompromisinglyhard work. I have no illusions about the difficulty of successfully completinga PhD, I only ask that you give me the opportunity and help to do so." There's a trick to writing SOPs, in that you have to demonstrate your passion, your skills and your research experience/interests in what you write. So just saying 'I will work hard' or 'I have a passion for this' is no good. What you should say is something along the lines of describing your REU experience and your interests in such as way as to convey just how important this is to you. So in other words, don't say that you will work hard, but show it in how you worked for your REU. Show your passion for the subject by detailing challenges you faced on your REU and what you did to overcome them. Show that you are a problem-solver and someone who is committed to pursuing your goals.
2) As for the other REUs, they may not match with your current interests but they can show how you work in a lab setting. Write about how you overcame a challenge or solved a problem on the project. Show how you benefited from the experience and how you contributed to the research/project. REU experience, even if not related to your current interest, is an asset - don't hide it.
3) I think I see this from a different perspective. I think profs are looking for people that match up well with what they are doing, so the more detailed you are the better your chances of getting in would be. Just make sure that the prof is looking for new students to join their group/lab (if its close to the deadline it may be too late to ask the prof though). I think that admissions committees expect that what you write in your SOP will not limit your interests over the next several years, in fact they may expect that your interests will grow/mature or lead you in another direction as you pursue your studies. What I do think they expect (and I'm no expert) is to see an SOP that is focused - they want to see that you can outline a plan of study, even if you it's not necessarily the exact one that you end up pursuing.