I like most of your post (some of it very much indeed!) but there's a couple of points I'd run aground on were I at your table:
Nope. For me, it's anything goes as long as it stays in character. It does take a less-serious approach, though, where one is willing and able to laugh even when one's own character's is getting the short end from others in the party.

And you have to be ready willing and able to sometimes give back as good as you get.
The only proviso is that it has to stay in character. In character, Falstaff and Jelessa can scream at each other until they're blue in the face but the moment Joe and Mary start arguing in meta-terms it's shut down hard.
I've tried this in the past but quickly ran aground on the problem of pre-declared actions no longer making sense by the time the action would resolve. A simple example: if your pre-declared action is that you attack the Orc in front of you but that Orc runs away or gets killed before your turn comes up, attacking it no longer makes sense and it's quite reasonable the character should be able to do something else e.g. move to a different foe and attack it instead, or chase the Orc that just ran away.
If I know the two characters share an obscure language I generally assume they'll be using that for such asides, and unless the king or any nearby courtiers also happen to know that language then there's no consequence.
That said, in my game the players would usually pass a note for something like this instead of saying it out loud.
Agree on most of your ways to handle this stuff. Yes, a humous handling is morec lax and is often the default, with "dramatic serious" encounters not the norm, is best. That creates contrast and prevents everybody ending up taking oneself TOO seriously in the long run.
Yes, the "be willing to have fun and roll even when bad things happen to your PC" is a must. But somer playeers sensibilities arec ata bit fragile so I prefer to make sure the players aggree that they are entering a potential can o worms intra-party conflict, and upp to what li9tthe llimits are, rather than waiting until something blows up to "finally realize" that it wasn't all a-ok. Personal preference here.
"Suddenly player X is super pissed off at player Y and the game table fun is ruined for hours on end if not the entire night, maybe even with potential permanent souring" is a way, way harder to "fix/stop" problem by the DM, aned otoo often it led to one player leaving. That just screws up the table dynamics to much just to satisfy one player's uncontrollable urge to have fun by being a dick or a tourette syndrome star member, whhich he picks up with another player soon afterwards. Better to kill the behavior in the egg, IMHO.
Predeclared actions (round start: everybody declare actions, then resolution in init order), were never a fav of my players. They agreed with the concept but found it way too "confusing". What I did back then was use PLAYER STATED ACTIONS/INTENT i.e. "I attack the orc!" and the orc movesc away? Treansformed into instant pursuit. I also allows a pure WIS check DC 15 to "switch on the fly to another completely different action" (DC 10 or no check needed if it was different but "reasonably similar" like say "oh the orc moved away? Well I'll just bash the goblin instead."
Eventually we dropped that way and the predeclaration is at the beginning of the player's current turn, and by default the resolution always try to match the stated intention. It still forces interesting yttacticall choices liike "Do I make 2 attacks on that orc, making pretty sure I will "down" it this round, albeit risking doing overkill damage, or do I split my attacks, risking downing
NO enemy this round?"
I understand thiss makes PCs generally weaker, but I also tennd to use less strong foes, too. I use Milestones XP based on "accomplishing the mission", so the players don't lose out. In facxt many encounters they solve it in non-combat ways.
Also, most enemies are not mindless monsters with zero sense of self-preservation. They don't exist "just to be killed", they have actual goals. And there is a "social tradition" that even (most) evil NPCs tend to respect a lot: Atv any time n a combat, you can surrender by dropping t your knewes, hands ii nthhe air, droppping your weapon. Thus means the victor won, but can't kill such a prisoner. If the prisoner betrays that "pact", though, and tries to rejoin back inti the fray, or escape, or fight back, then he DESERVES death. And tongues talk: PCs can ellect to ignore this "right of surrender", but it will become know that tey are corrupt and foul, and gain noot only a bad reputation with more honorable people (thus most noble NPCs and other NPC good guys, and at some ppoint even ordinary NPCs willl start to shuhn vthem), but bad guys will also stop respecting the PC's "right to surrender".
Most fights vs intelligent creatures end wayyy before "almost all enemies are downed". That pack of 20 wolves? Unless they are totally mad from supreme hunger, as soon as 3 or 4 woolvves are easily dipatched while the party still hasn't got a scratch, morale check, and on a failure most of the beasts just decide to smelle where thiis is inevitably going and decide to flee. The few "more hostile and bloodthirsty and violent" stragglers wisen up the next round.
So we have quite a variety of fights. Fanaticallly drugged cultists will probablly fight to the death. But plain bandits? Oops those PC travelers seem to really know how to fight back, time to ESCAPE! Even if the bandits would ultimately be 100% certain to WIN, they put more value in making sure they survive to fight another day, rather than "Ok
only half of us will probablly end up be killed but we will be certains to kill off all the PCs". Each and every bandit 100% values making sure his own life ain't risking a 50% death, WAY before valuing the other bandits, lives, or "killing the PCs". They aren't mere bags o' hit points, fighting the PCs ain't their "goal in life". It's more like gaining easy gold from weakker non-resisting and intimidated targets. No non-ultra-stupid creature has a death wish.
Of course, for raw actual beastly monsters, anything goes, with no repercussions.
We also have the "Parley" thing. This has to be done BEFFORe thhe fiighting stats, and if accepted, typically means the parleying party will not be the one initiating a fight, and try to negotiate some appeasement and settlement, with the understanding that the group accepting the parley ALREADY starts the negotiations having done a favor to the side asking for the parley (thus some kind of payment will always need be involved). BBrrakking the truce of a parley is a SERIOUS offense, tarnishing reputatriion for a long time.
And even if the PCs decide to "leave no witness behind", their group reputation wuld still be tarnished anyway. Because the gods saw it, and they favor falls upon heroes keeopoing their word true, and their divine disfavor falls upon those betraying that thrust. Somehow, somewhen, words get out. A bird saw it. Then a god make that bird go tweet the story to a local druid. Who later went to buy soome booze at a local tavern, and the NPcs there randomlly starrted tallking about the Pcs'" nmoble and heroic deeds", with the druid not failing to dot their I's and cross their T's. And thus, thre part y becomes know and dangerous and unreliabler bloodthirsty mercenaries, instead of as heroes. -1 Reputation with "Party Honor". Which then taints all future social interactions asc a check penalty with NPCs for which honours might be important. Thus also includes the gods plus any encounter in which the PC might use the same Honor Codes to survive a fight they would otherwise really badly lose instead.