Should I turn down PhD offer if I think I may not pass qualifying exam (risking immigration issues if I fail)? I am done with my MS in EECS from the University of Michigan-Ann Arbor, and I currently working full time in a good company on OPT.

I will be defending my MS Thesis in coming December. Moreover, when I contacted one of the well-renowned Professors in my department he expressed his interest in taking me as PhD student depending if his funding is approved in next few months. 

Here is the issue:

My overall GPA is 3.43 and my core GP is 3.5. I had already mentioned this to the Professor, and I straight away told that after working hard I am just a solid B+ student that too when I study with friends. I sent my transcript to him thinking he would turn me down based on my GPA, but he did not. 

My question is: I don't have issues with quitting the well-paying job for the PhD program. But what scares me is the oral qualifying exam. If I don't make through the exam, and they decide to kick me out, I have no options. I will no longer be enrolled in the PhD program, and cannot go back to the industry as I have used up my OPT.

My future goal is to join academia, but if I don't pass the qualifying exam, I will put my family and me in immigration issues. So, least I can do is to work in industry and give up on joining Ph.D. At least I will not put my family at HUGE risk of getting kicked out of the US after spending so much on MS program.

Please advise. The reason I ask this question is that one must aware his limitations. When I compare myself with fellow PhD classmates, they way better than me in course work. I have never met any classmate in my graduate school who are less smarter than me.
Professors and admissions committees have a lot of experience in accepting students- they wouldn't offer you a position unless they were confident that you were capable of handling the program. It's just as bad a situation for your professor if you fail at the oral exam stage: they will be stuck in year 2 of a 5 year grant without an experienced student and no immediate replacement to carry on that work. 

Two specific thoughts:


Find out who the Graduate Program Chair or the Graduate Admissions Committee Chair is for your department and ask them the same question you asked your professor. These are people with a lot of experience handling and admitting graduate students, and they can give you an impartial opinion.
See if this program has an alternative to the oral examination. My program allowed thesis-option MS students to waive the oral exam under some conditions.  In some labs it was very common for students (typically international students with weaker presentation skills) to fulfill the MS requirements and write a thesis rather than doing an oral presentation.