OK i'll give this a stab. Firstly as a Amateur, I don't want to burst your bubble, but this piece looks quite advanced, so best not to underestimate the workload that is required here.
1 - the painstaking process of learning this piece without reading music will double your workload (unless you were an autistic pitch perfect genius) and so I cannot ignore the fact that I recommend you learn to read music if you want to learn this piece.
2 - If your goal is to play nothing but this piece, I would consider breaking it down into exercises and studies that relate rather than try and tackle this entire piece from start to end, which brings me to your first point.
DON'T AIM FOR 100%!!! lol. right gone with the caps, playing the 1st 10 seconds at 100% speed is pretty pointless.
You should be aiming for 50% speed at all times until you play the whole piece fluently. My first suggestion is don't go to the start of this piece play 10 bars, it gets really hard and you give up when you ultimately get to that difficult part that gives you that "discomfort you mentioned"
I'd definitely suggest going straight to the hardest bars and learning those bits, not only will it give you a realistic idea of how much time will be required to learn the song, but it also prevents the typical trap pianists get into when learning a piece.
You start at the start (usually not too difficult) you get a few bars done, it's quite nice, you're at full speed!! then you hit the difficult bit, it sounds bad...you go back to the start where it sounded nice again, you play through then it the same bit... it's a little better but still BAD. you get my point here? You only really get good at the easy bit at the start and you give the easy bit far more practice than the difficult part.
Next the GYM and the PIANO are very different. In the Gym you're goal is to get bigger by over working muscles, tearing or damaging them, replenishing them with food or supplements and then going again. (I know as much about the gym as I do the piano

)
Your fingers aren't lifting weights, you're not supposed to be tearing anything here. Your muscles tire yes and you give them a rest, but there should never be injury or major discomfort. You've also partially answered your own question. When you go to the Gym do you do Biceps non stop, no rest for 1 straight hour? ... unlikely

we do different muscles, we have rests, we grab some water, etc, apply the same to the piano, take a break every 20 minutes, grab a drink, change the piece of music to stop stressing the same muscles in your hands, practice hands separately so you can give one hand a rest while trying another, there are lots of options here.
Now, understand there's a difference between pain and fatigue. Fatigue is simply you tire out, trill 2 fingers as long as you can to the point you can't no more, you will feel fatigue, it's not the same as pain. Pain is incorrect movements, it's a stool too high, it's your wrists too curved, it's your shoulders to tense, as one incorrect motion invokes another to over compensate you can be lead into believing that's the problem. If there is a particular motion causing trouble, trying it in your right hand and see what it does differently, and see if you can mirror in your left, slow things down until it is comfortable.
Technically there is no limit on how long you can practice for, if your motions are correct and you feel no pain, one professional may advise 2 hours and another will tell you they played for 10 hours, but what you will likely find is every hour even every 15 minutes that go buy that you are sitting there at the piano, the return of your studies will start to diminish.
You have to fully engage in your practicing and judge yourself how much you can practice before your brain burns out. On the weekends I probably do around 5-6 hours, but I break that down until 20-30 minute sessions with clear breaks, and clear goals each session. If I purposefully learn something that I forget the next day, I cut down the sessions. During the week Probably 2 hours a day same breaks.
Lastly I want to go back to tempo again as I am concerned about your thought process here. 50% is the fluent speed I suggest when learning a piece, learn the notes at a slow speed with aim for 50% of the final, and only when the whole piece is learn through step it up, but don't think you can go 50-100% and get everything perfect. Patience here, if your full concentration is dedicate to the notes, the phrases, the dynamics, the tempo will pick up naturally and it won't even be something you realise.
Summary
Get a metronome and learn to read music rather than watching the Synthesomethingorother
Focus on the most difficult parts of the song for a realistic approach and manage your time
Use studies or exercises to even out your hands ability if you are struggling with particular parts
Injury / Pain is a NO NO NO - post a video here of your playing if you want more advice
Have set goals and little practice sessions as to not over do and mindlessly bash notes
Always aim for a slower tempo, focussing on the sound rather than the speed. The speed will naturally increase without thought, but the sound will not.