Hello Julian,
What are you hoping to achieve or make progress towards? The process of the content of your email seems to indicate that you have a desire to progress and that perhaps if you find the proper combinations of conditions you will achieve something. There seems to be a general theme of desire for "control" in your post that you would like to find a way of controlling the progress by setting up the right conditions. I have gone through that process myself. I find it to be a process of contraction, selfing, identification and the unsatisfactoriness of it shows itself to me more and more, especially on retreat. I like Goenka's advice for intensive practice on retreat, very simple and to the point he continues to tell people to be patient, diligent, and persistent. Basically, just keep practicing continuously. Thats the ticket. Just keep practicing and the progress that you are hoping for will happen.
That being said, personally, I do alot of reclining meditation when sitting gets too intense on retreat, to manage pain that is distracting, but pain can also be helpful to work with and progress by observing it. I generally eat less on retreat as I find that when I eat less, I become less lethargic and it helps my practice. I remember a dharma talk I listened to a while back by Shinzen Young which may address your thoughts on extended sitting.
https://www.youtube.com/watch?v=gYSSf71Vo7w.metta,
Drew
