SadMIL I know your behaviour comes from a place of love for your son BUT please take a step back and see what you are potentially doing to his marriage.
When you say "for example, I messaged my son asking to come over to do the gardening..." it sounds like you want to do this. You aren't even just wondering if there is something useful you could do. AND you are only communicating with your son.
Likewise the laundry. You have decided that they need help and are trying to sort this out with your son. She has "put her foot down". No wonder. You didn't even offer her the help (of her choosing), you are again cutting her out to try to get your way.
I know it is hard to watch your family struggle (as we may perceive it), but they have to be a TEAM and we, as grandparents are not part of that. We want our children always to have the best, easiest, most satisfying lives possible but they really can't do this if we interfere. We have had our time with little ones and now it is theirs.
Please think about this - if you drive a wedge between them will your son be happy? Will he love you more (or less) for causing pain? In future, if you have time to spare, message his wife and offer "is there anything useful I can do?" Let HER take control of her own house.
You do say that these problems didn't occur before the grandchildren came along. With consideration you can get back to that happy place.
I really hope you do.