In DnD a man can swim in full plate armor across the mississipi river, with no discernible practice, training or exerted effort.
In DnD, a scholar who has never picked up a pack in his life can slug on an 80 lbs backpack, take an 8 hour march across rough terrain, go to sleep, and do it again the next morning. For a month.
In DnD, your average Knight is as strong as a silverback gorilla, which in the real world is considered about as strong as 20 average mean. Gladiators are stronger still.
In DnD, a street thief with no training, can free-hand scale a 9 story building in six seconds. Everyone can do so in thirty seconds.
In DnD, everyone from most every dimension speaks the same language, with barely any regional variance.
In DnD, everyone across dimensions uses the same coinage value. A gold piece in a poor village in Flnaderes is the exact same value in Sigil or in Strixhaven academy.
There are more "exceptions" to DnD being like the real world Earth than there are things modeled after it. The closest things are really "it is a planet with water where people have jobs"
1. A man has an average STR 10, well below the STR 15 requirement, so his speed is 20, making his swim speed 10 feet. The Mississippi river averages about a mile wide (let's round to a nice 5000 ft). The Mississippi river also has the strongest average current of any river in the U.S. at almost 600,000 cubic feet per second. This would most certainly require a Strength (Athletics) check to make progress. Oh, and given these parameters the water would also be difficult terrain, further reducing speed to effectively 5 feet.
Now, your claim is with "no discernible practice, training..." so we can keep the DC at 20 (hard) given the conditions, but at least his can make a check dispite having a +0 modifier to the check (no proficiency since no practice, training; and no STR modifier).
Given 5000 feet at 5 ft/ round means 1000 checks to "gain any distance". 1000 checks for DC 20 Strength (Athletics) at +0 would average of 20,000 checks to swim the distance. At 6 seconds per round, that is 33 hours and 20 minutes of swimming. Which of course brings exhaustion into play, which with a CON 10 and no saving throw proficiency for the average man, we can expect the first level of exhaustion around hour 10, which imposes disadvantage on further Strength (Atheletics) checks. At this point, the man would be less than one-third of the way across. With disadvantage, the remaining 14000 checks for DC 20 become 280,000 checks. At this point, it would take over 19 DAYS to make all the checks required to finish the swim. Exhaustion would kill the man long before that point could ever be reached.
2. Sure. A scholar would probably be Intelligent enough to distribute the weight evenly, use a walking stick, rest often, etc. For many days he would likely be sore, but he could do it.
3. Well, we don't have stats for the silverback gorilla so we'll never know. Besides, such claims are irrelevant and contradictory. One site claims the 20 average men or whatever as you do, and on the same site says 4 to 9.
4. I guess you must mean the subclass "thief" so you have a full climbing speed (assumed 30 feet). Which is 3rd level, and certainly not a rogue with "no training". Otherwise, much depends on the building. Few handholds would demain ability checks as well. Failure might be no progress, or might be actual failure.
5. Is that how it works in real life?

(j/k)
6. Incredible coincidence, huh? Must be
magic!

(j/k)
That was fun. Anyway, we all know D&D (as presented) is not a simulation, but a game. That game can be based as much on "medieval Europe" or whatever as you choose, or could be completely foreign. But we shy from that in general because for most groups to embrace the "fantasy" it must be grounded in "reality".
He has skin stronger than enchanted steel. He is stronger than a rhinocerous at full charge. He can reach inside your mind and twist your worst fears into existence just from you being near him. His eyes can pierce any disguise and his voice echoes in your mind like black smoke. He is tough enough that he laughs off shotgun blasts and grenade. A missile strike MIGHT injury him severely enough to force him to retreat, if it even hits him, since he is faster than a horse at gallop
By the way, this is just a standard Pit Fiend, not even a named devil.
There is some (very well described!) narrative license going on here...
AC 17 (without DEX +2) is not stronger than enchanted steel, since non-enchanted steel can give you AC 18.
Pit Fiend STR 26, rhino STR 21. Both are large creatures and who is "stronger" at any moment would depend on the contested Strength checks. With a +3 net advantage, the Pit Fiend would "win" about 60% of the time. "Stronger"...? In an absolute sense, sure.
Yeah, it can make you frieghtened, has truesight, and telepahy. But he isn't faster than a horse (fly speed 60 vs. speed 60 for the horse...).
At any rate, there is nothing really "standard" about a Pit Fiend. I mean, honestly, come on. Maybe something a bit more common than a CR 20 fiend, arguably the most powerful non-named fiend in the game.
But hey, in a world of magic, a martial just
needs magic to combat such a monster, they don't have to
be magical.
