The mental exhaustion from practice is unavoidable if you have a dead line with a lot of work to get through and especially if you are unmotivated. The physical should never be a problem unless you are playing absolutely insane music which is simply not the case all the time unless you are crazy and even then "difficult" music can be practiced gently.
You can pretty much practice everything in the p range. However with some actions at the piano it doesn't matter if you play loud or soft, so I will not overpractice things that can put stress on the hands in this way such as, very large stretches, glissandos, or finger twisting multiple simultaneous note patterns in one hand etc.
How you practice also depends at which level of ability you are at with the piece. If you are confident that you can put whatever expression in any situation of the piece at your own will then it becomes irrelevant to practice pieces with expression and more efficient to simply learn all the notes.
I find this true when studying my students pieces, many of them are much lower than my own level so when I practice them I simply practice the notes, the expression is obvious. The memorisation of notes is also obvious but not as automatic as putting in the expression, it always requires some focused attention. Once the notes are controlled the expression can automatically go over the top. This can only be done with pieces which you find relatively easy.
If you are not confident you can crack the expression you should practice with the expression, this however will limit the amount of time you can put into a practice session and how fast you progress through a piece. When you practice the expression you can get caught up in confusion and negative thinking; Oh, this isn't good enough, I must practice this one section 100 times before I move on, is this completely correct? etc. When practicing notes you have always a limited number of repetitions until you achieve the memory, but with expression we can go on forever with repetitions thus waste a lot of time/energy.
So one question we should always ask before sitting down to practice; Could I play this piece with expression if I knew all the notes? If we know our hands well we can always answer this without even touching the keyboard. Practicing expression saps our energy especially if a piece is loud and furious. Knowing you can produce the expression at will and holding back that potential until you have all the notes mastered will reserve your energy.
When I teach a student who practices an energy sapping part of a piece (the definition of what is energy sapping is different for all people) with me I will first ask for them to play it with expression (even if the notes are not 100%). If we notice that producing expression is controlled then we abandon the expression and focus on the notes if the notes are not completely accurate. It makes it physically less taxing for them and focuses the effort where it is needed most. Expression is mostly controlled by what we hear, so long your ear knows what is good and what should be avoided then your expression is more often than not than fine.
After you ask a student to repeat say... a Rach FF over and over again they will eventually get exhaused and unmotivated to produce the big sound and thus your idea to teach control of the expression and notes will collapse. Hearing the same big chord over and over again can give you repetitive stress syndrome

You will eventually find you don't care about the expression because it is happening again and again and boring your ears to tears. It looses the effect it should have on you.
When I teach expression I don't care if they hit a wrong note, all I care about the expression. When I teach notes I don't care if it sounds like a robot, so long all the notes are correct. It is asking a lot from a student to play all the notes and expression correct when
practicing a piece and I think it is an ineffective method of teaching unless the student is studying a piece that is very easy for them.
If you find you have to repeat a section many times you must question is it a note or expression issue. It is always one or the other and many more times note problems. To mindlessly repeat trying to maintain notes and expression is silly and will take you many more repetitions to come to the solution, and of course make you tired both mentally and physically.
Also I have found studying multiple pieces can keep the mental stress down because once you have had enough of one piece you can refresh yourself and move onto the other piece and make some grounds there. Then as soon as that second piece annoys you go back to the first or even a third one depending on what you know you can handle.
To give a piece you studied for 1 hour a break then coming back to it soon after you spend time with an other piece will ensure you take stock of what you remembered and what you found you forgot. YOu take away that chance aspect of forgetting your notes which can easily happen with long practice sessions. Your brain starts memorising only for the short term (while you sit and practice) but as soon as you stop practicing and do something else, when you come back you find you forgot chunks here and there. THis definately add to your frustration and mental stress!
You must then put in memory aids (which is a whole other discussion but basically reminders on the sheet music) to help you recall things you forget. The stress level of forgetting stuff is limited then because you are taking stock of everything you forget and controlling it.
The memory aids you have on your sheet music will act as a reminder of what you need to play, don't keep these aids in your head, what notes escape you must be marginalized from the rest of the music and you have to make a written statement which forces you to remember it in context with the music. This way you can practice the piece without the piano but by simply observing the memory guides and reaffirming with yourself what needs to be done.
Once you have made several sucessful repeptions on the keyboard and achieved the beginnings of the muscular memory then you can forget about written memory aids. But these aids will always benefit you especially if you don't play a particular piece for a number of years then decide to learn it again, all the infomation of what troubled you before will be there to help you out and reduce the stress level that might come if you forget a piece.