I haven't read this whole thread, but I am in a similar situation and our time lines are pretty close, too. My stray showed up in May and I had him neutered in July, so a couple of months before PiPi. Jaeger had to stay outside because of my two indoor cats being very resistant to a new roommate, and I would always be worried when I got home from work that he would be gone. At first he would go away for a day or so, but after he was fixed he would always be around after work...as long as I wasn't late. If I was late he would be gone and I worried until he showed up again, usually right before bed. Later in the fall, even if I was late he would still be around...not waiting on the porch, as usual, but out in the grass, figuring he would have to hunt for his dinner...until he saw me and realized the miracle can of food was going to be opened very soon.
So I know you are worried that your PiPi may leave when he is allowed access to the outside again, but since you had him neutered right before winter, his hormones are slowly ebbing and his desire to wander, get in fights and mate are leaving him. It is actually a good thing that it is winter, because no cat likes to be cold and every day he is inside is another day for him to get used to the high life and know that you will feed him and keep him warm and that your dog will not harm him.
You are doing great with him, and he knows it...
So I know you are worried that your PiPi may leave when he is allowed access to the outside again, but since you had him neutered right before winter, his hormones are slowly ebbing and his desire to wander, get in fights and mate are leaving him. It is actually a good thing that it is winter, because no cat likes to be cold and every day he is inside is another day for him to get used to the high life and know that you will feed him and keep him warm and that your dog will not harm him.
You are doing great with him, and he knows it...