Thank you for the review. I do recall the details now, but since we had a phone conversation previously, there was nothing to remind me.
I understand you concerns for quality of life for Max and even though the drugs mentioned are often tolerated well by many dogs, with the rate of recurrence that you have already seen, I think the risk versus benefit is too high. It sounds as if you making the right decision.
As far as how long does he have, it is really impossible for me to say, and that would be my response even if I had him there in front of me. You really have to take each day at at time. I would guess that as the end approaches, you will note the same cycle that you have been seeing, but that he won't start to improve or his recovery will be much more prolonged. Diphenhydramine can be dosed at 1-2 mg / lb and can be given every 6-8 hours. At the higher end of the dosage is where we usually see drowsiness, so that may be a good way to help calm or sedate him as things progress.
Unfortunately, it is not possible to give the euthanasia drugs orally and expect predictable results. I guess that would be the challenge. It is still possible that you would be able to arrange the send off that you have in mind (which sounds lovely, by the way), but you would need to arrange to have a house call vet met you at the house. This may take some calling around if your vet does not provide this service, but it is getting easier to find in many communities. The other challenge is the timing. You will know when it is time, but it is not always a convenient time, especially when you have a plan in mind as you do. He could go on as he has for months yet so it is just impossible to say, but I would start to make some calls so you hopefully are ready when the time comes and then things will run smoother.
As far as deciding when it is time, what I always tell my clients is to look at their pet and decide if they are still getting any enjoyment out of their day. Most owners know their pets pretty well and know the things that, through their life, made them happy. Maybe it was a special toy, going outside, greeting their owner, etc. If they are still experiencing those things, then let them continue until that goes away. Most pets, in terminal stages of discomfort or mental decline, get no joy from the things which previously had an effect and begin to with and begin to withdraw into themselves. If that is the case, then it is probably the right decision to intervene. As long as they are alive, in your heart, it will never seem like the right thing, but there comes a time when you realize they are really no longer here anyway.
Good luck with your upcoming time. Max is lucky to be in such a loving family.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.