You have around a 65% chance of acceptance to one MD program with a GPA of 3.5 and a MCAT of 35. The 5 years of volunteering will look good on your application: most applicants only have 150-200 hours or so. Schools with a service mission like Creighton would probably like that.
I would apply to about 14 low-tier MDs, 3 reach mid-tier MDs, and 3 DO schools that appeal to you. You are a strong candidate for DO: your stats are all great and they would get excited about an applicant with five years of volunteering.
No the way the engineering curriculum works out at my school is that next semester I have to take basically all engineering classes and a couple of humanities. I tried to squeeze in something, but it didn't work out.