Re: Bad poop!
I think the 2 most likely causes for the green, frothy poops are 1) the once-a-week unnecessary formula or 2) some sort of mild gastrointestinal illness (and I say mild because you don't mention any other symptoms). Either way, this is not a big deal: green poops are in the spectrum of normal for poop color, and are not in and of themselves health problems unless accompanied by other issues, like poor growth.
The poop issues you describe are not constipation. Constipation poops are hard, dry, and will roll right off the diaper if you hold it up by one end. They are also infrequent, and 4-8 poops per day is NOT infrequent! The discomfort and straining related to pooping is a little suspicious, but I'm guessing it will clear up in time, particularly if you stop grandma from giving baby formula!
I have to ask why you're leaving your child with his grandma when she blatantly disregards your instructions about baby care. Giving baby unnecessary formula would be a complete deal-breaker for me.
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"