All of these seem pretty straightforward. Someone actually declaring combat starts combat. Then it's potentially a roll for surprise (or implementation of one already made), and then initiative. Except the last example, that includes a conclusion past the initiation of combat - that has to be worked out.
So the example with the negotiations break down but no one wants to start. It doesn't transform into combat until someone actually starts. A declaration from either side. It seems everyone knows it's coming so no real chance at surprise so it's just initiative.
To give a related example, say there's negotiations that aren't going well but it doesn't look like hostilities are imminent, and then the poker-faced assassin goes for a stab. In that case I would do surprise but with Deception vs. passive Insight.
Let's look at Example #4, the ambush. We've got the stealth checks and passive perception going. The party wants to coordinate - is this some unfulfilled wish floating in the aether or did they actually plan to do so by setting up a signal, even if it's "wait until I fire". If they had no plan for coordination, it is completely possible that some of party are caught off-guard as well.
We already know what the result of the stealth vs. perception check for surprise is, since the ambush-ees didn't see the party (else it would be different). So it's likely something like "They are riding into view around the far bend. Stop me when you want to do something. Okay, all are in view and about 200 feet from the closest of you. 150 feet. You can see orc warchief, he's leading from the front. 125 feet. Let me know when you want to do something. 100 feet. 80 feet. If they get within 30 feet it's more likely they will spot you. 60 feet -- what's that, you want to attack when they are 35 feet from the closest? Gotcha."
Now, by the book, initiative will determine the order, and the people before the person signalling will be readied since that is their effective declared action. But mechanically that will usually penalize the people who rolled well (no move+attack, no extra attack, etc) which is contrary to the spirit of the resolution system so if it's "wait for me to fire" I'll put them at the start of round regardless of initiative.
(Note for players with DMs who just roll initiative and don't want to chance getting penalized with "Ready" - make the signal something you can do outside combat, such as the cry of a native bird. That shouldn't be part of the initiative order, so then veveryone just goes when they roll.)
Example #5 is only slightly about the declaration of combat, and that part has been covered. The rest is about ensuring one specific outcome, and really for me that's not something a game like D&D does. Other games you can lay out stakes and go for it in an atomic transaction, D&D wants to play out each bit.