Guilt is only something we can put on ourselves. Others can do their best to make you feel this way, but only you can actually put yourself or allow yourself to slip into that mode of guilt. Everything about your in-law’s relationship irks you so you must honor your feelings. You must be a person who is caring to a fault, the fact that you stress yourself and put yourself out just to keep peace with others, shows this all to well. To me this is not healthy, I have been there and done it myself and it truly shows weakness. I am not saying you should not be kind, fair or even generous, just not at your expense or anyone else’s.
Pull back from your in-laws but always leave the door open. Let them know they must contribute to your daughter’s life in their own way and by initiating contact with their granddaughter. She is their last physical link to their son, I can not imagine why they would not take more of a lead in doing so.
I think you must be clear with them, they could be loving but simple people and they could miss read your generosity from over the years as the way it has always been and must remain. Tell them how you feel and be honest. They will either get it or they won’t but at least you can feel comfortable in knowing you said what needed to be said.
Bottom line don’t knock yourself out trying to make the relationship between your daughter and your in-laws perfect. All I would ask is that you never shut her off to them, but they must take some initiative in the matter. I wish you well and what you are feeling and experiencing is common in a lot of families where one spouse dies. The whole dynamics change and that is as it should be. You even said your husband was pulling back before his death, sounds like he was leading by example.