First let me welcome you to the board! We are by no means relationship experts, but we will always give you the benefit of our personal experience.
As far as your very detailed situation appears, it kind of appears to me that your lady love has been "getting her ducks in a row" to exit your relationship for a while now. You have to remember this isn't her first trip to the "break-up" rodeo, since you mentioned she had been married before for twenty years. Anyone that goes through that kind of divorce is going to know exactly what she needs to get lined up for herself well before leaving a longterm relationship, so she doesn't end up on the curb with nothing for all those years.
Step one was clearly purchasing the "investment" property a couple of years ago was a warning sign. Renting it out until she wanted to or was prepared to leave the relationship was obviously something she learned from her divorce...have an exit strategy. I'm sorry you feel the stress of her leaving contributed to your having your stroke. However, it was an "inconvenience" to her plans for leaving on schedule, but at least she was decent enough to stay and take care of you for a while. It's clear that her feelings for you have changed, since following HER operation, she would rather have her sister taking care of her instead of you. That is step two. The fact that you had separate bedrooms should have also given you a clue. Letting her sister in on the proposal plans was foolhardy, especially if you didn't have your plans nailed down, as a sister could only keep that information to herself for so long. You seemed to be more concerned about making a big show of the event than the sentiment and the commitment the proposal represented. You also mentioned something about the timing not being just right...how many years did you have to find the right moment? You made it all about YOU, what YOU wanted, when YOU wanted to do it, etc.... In fact, your entire post just screams "it's all about MEEEEEEE!!!!
Your poll about how long you should go before contacting your "lady love" is delusional, like if you pick the appropriate amount of time, she'll come running back into your arms. To quote you, "This is MY theory and MY decision to handle it this way." Yeah, that may be true, and I hate to say it, dude, but she's history. Your statement about her new digs being 70% smaller and not having a garden is absurd, like those are supposed to be a negative things that make moving back in with you so much more attractive? Good grief man, she's the one that bought the place herself, so she knows exactly what she's getting.
I really try to be sympathetic when it comes to people that are hurting, especially when it comes to affairs of the heart, but frankly, to use a football analogy...It's fourth and forty, time to punt. You are out of time and yardage man... acknowledge your failures and move on. She's not coming back.