Why do we assume that PHB rules apply to monsters?

The party is deep into my 5e-updated classic Greyhawk Giants series.

They are returning the body of a stone giant killed in the siege of Headwater to the stone giant’s clan. The Thane of the Stone Giants intends to hold Funerary Games in honor of his slain kinsman and invite the PC’s to participate. One of the games will be unarmed combat.

In calculating an unarmed strike from a stone giant, I understand that the damage from the strike will be 7 points (1+ the stone giant Str mod). For the attack roll, however, I was unsure of whether to give the stone giants their proficiency bonus.

This question about unarmed bugbears, this question about unarmed skeletons, and this question about unarmed Flameskulls (?!), all have answers which state that monsters get to add their proficiency bonus to unarmed attacks, with the last question containing an answer from luminary SevenSidedDie stating as a general principle that "creatures always have proficiency with their unarmed strike".

As justification for this position, these answers cite some variation of the Player’s Handbook, PHB errata, or D&D Beyond, quoting "You are proficient with your unarmed strikes." None of the comments question this justification, and some support it.

Perhaps it is because I came to 5e from earlier editions, but my working assumption is that monsters / NPC’s have stat blocks, not character levels, and don’t necessarily follow the rules in the PHB. I read "You are proficient" and intuitively feel like ‘you’ means ‘You PC’s’, not ‘All creatures’. I don’t see anything in the Monster Manual that says that monsters in general follow the rules of the PHB or are proficient in unarmed strikes; I do see sidebars noting that monsters are proficient with their "armor, weapons, and tools" and that grappling rules work differently for many monsters compared to PC’s.

Under the general principles of "there are no secret rules", and "abilities do what they say they do", where can I find a general statement saying that monsters follow the rules of the PHB, and that such "you" statements apply to them as well?

Related: Are monsters subject to the massive-damage instant-death rules?

Related: Is the telepathy rule in the Monster Manual only applicable to monster telepathy abilities?

Are the monsters in the MM actually following the monster creation rules?

The guidelines for making your own monsters are based around the Monster Statistics by Challenge Rating table on p274. Inspired by this question regarding Animated Armor (MM 19), which is CR1, let’s see where the Armor would be if we made it from scratch…

Defensive CR: The Armor has 33 HP, but step 9 (Damage Vulnerabilities, Resistances and Immunities, DMG 277) has us adjust effective HP based on resistance.

So we start with 33 HP, giving us CR 1/8, which tells us that we’re using the first line of Effective Hit Points Based on Resistances and Immunities, for an x2 multiplier for each Resistance or Immunity.

The Armor has two Immunities, so has 33x2x2 or 132 “effective” hit points. That puts us at CR 5. We then look at its AC of 18, which is 3 higher than the normal 15 for CR 5. That adjusts us up by one, to CR 6.

Offensive CR: It does 10 (2×5) points of damage, for a CR of 1. Its Attack Bonus is +4, which is 1 higher than the normal +3, but doesn’t adjust CR up.

Final CR is the average of the two, or (6+1)/2. That gives us 3.5, which we round against the players, for a final final CR 3.

How is Animated Armor a CR1 challenge? Its only notable disadvantage (from a combat perspective) is its Antimagic Susceptibility, but that has no effect on CR, per DMG 280. What’s knocking it down from CR 3 to CR 1? Or am I horribly misunderstanding how the creation rules work?

Are monsters with Sunlight Sensitivity supposed to have disadvantage when attacking or perceiving a target that is in direct sunlight?

Certain subterranean-dwelling monsters, such as drow and duergar, have the Sunlight Sensitivity trait, which is consistently worded as follows:

While in sunlight, the [monster] has disadvantage on attack rolls, as well as on Wisdom (Perception) checks that rely on sight.

Of course, drow and duergar are also playable races for PCs. These playable races have an identically-named Sunlight Sensitivity trait, but its wording is different:

You have disadvantage on attack rolls and on Wisdom (Perception) checks that rely on sight when you, the target of your attack, or whatever you are trying to perceive is in direct sunlight.

This is a significant difference. A drow PC attacking a target that stands in sunlight has disadvantage where a drow monster would not. One can imagine a drow PC traveling with a group of his monster drow kinsfolk and ending up the butt of jokes (or worse): "Poor Drew, for some reason he just can’t hit the broad side of a barn when we’re up on the surface. The rest of us, we just find some shade to snipe from and we’re set. Even that doesn’t seem to help Drew. I don’t know what his problem is."

Am I missing something? Has this difference been either explained or errata’d somewhere?

Are there any monsters or races that can cast spells without verbal components

So a while back I saw the piper race in their list of racial traits they have the following

Mystic Voice (Su): Piper spellcasters have a natural magic in their voice and must use it as an element of all their spellcasting. The additional magic imbued in their voice means a spellcasting piper never has to worry about somatic components, and ignores the somatic component of any spell. However, pipers cannot cast spells without singing out verbal components. Even spells that normally do not have verbal components gain them when cast by a piper, and pipers cannot use the Silent Spell feat. Further, a piper cannot cast such spells quietly. It is not possible for a piper to make a Stealth check when casting.

I was wondering if there was a race or monster that can do that but with verbal components. We have the obvious silent spell metamagic but I’m looking for something innate. My DM approves 3PP as well as Paizo products. Homebrew is on a case by case basis, and I don’t want to bother him over something that won’t come up right away since I don’t plan to use this character immediately unless I really have to. I can use characters that are half-monster allowing them to access 1 or 2 of their monster parent’s abilities as long as I’m not doing something like claiming they inherited the power of Cthulhu or something. Let’s say CR 4 max to give a hard limit, probably a bit low but better safe than sorry.

So my question is are there any monsters (max CR 4) or races (including 3pp) that can cast spells without verbal components.

How should i balance my monsters

I have created a few new classes and races for my game they all have the strengths and weaknesses but are still very powerfull.What is the best way to balance monsters or if I should simply put in more powerful creatures.My players are currently playing my campaign were they are traveling to the astral plane to kill an empyrean as a favor for having a devil save them in battle.I don’t want to keep throwing out powerful creatures but don’t want them obliterating all the weaker ones so how should I balance these monsters either give me tip on the best way to balance monsters or send me custom rules and monsters that can be fought at a medium difficulty.This game is in 5e