I agree with all you wrote. The older Manuel was definitely a different person. But, he could have been the father of the other Manuel. Even if the younger Manuel was really a little older, it is possible that the older Manuel was also a little older, too. They were really bad about giving the correct ages back then. Concerning all the kids with Harriet in 1880, I also think that they were grandkids rather than kids. I'll see if I can research them in 1870.
Have a good day.