I wonder if their decision would have differed had MM taken a one year maternity leave. I know some posters will be up in arms about this, but many women are more "hormonal" during pregnancy and thereafter (I know I was, and I also lost my outstanding memory when pregnant, enough that other women in my office joked about it).
As pointed out, that is a lot of change in a very short period, and, added to that, is MM's adjustment to the UK, which obviously, never occurred. But, I think she wanted to live a celebrity life, not a royal life.
I suspect Harry is going through that same homesickness right now, and it's probably worse, being cooped up in these extraordinary times. He may overcome that with time. With the exception of my husband, who not once longed for his homeland, I have never known any immigrant who didn't go through a period of profound homesickness, usually at around the 1 year mark in a new country.