I've taken the liberty of re-sequencing some of the quoted bits, below, as some relate to each other.
So plan for spots to get new players/ new characters in quickly. A player sitting out for anywhere near a session (much less multiple) is completely unacceptable.
This assumes the ability to predict what the PCs will do and-or where the PCs will go next, which is by no means an exact science.
If your group LOVES combat, well ok. But the point is, don't just expect to have a session long combat and not bother with other stuff.
Not sure what you mean here. If there's a major set-piece combat embedded in the adventure I-as-DM would rather plan for it (given the vagaries of PC actions as noted above) such that, for example, it can start and finish in the same session rather than either span across a session break or keep us up till 4 in the morning.
6. Because if it means collusion with 1 player without the knowledge ok of the other player, it impacts on their agency. That's usually bad.
Not sure on this - having a secret plot involving one player doesn't impact on anyone else's agency until-unless that plot starts restraining what other players can have their characters do. Now, I admit I've never seen a player plot with a DM to intentionally set up their PC to be a red-shirt; but I've seen - and been involved in from both sides - many other secret plots.
Flip side: I've many times seen players on their own initiative play their PCs as red-shirts, without any help from the DM. I don't think that's what was being got at here, but I could be wrong.
The point is, don't bore your players with long winded NEEDLESS talks. The streaming DMs are mostly doing it for the benefit of the audience not the players. The DM can spotlight hog too - and that's bad.
Again he's talking about spotlight hogging. Where one or two players dominate the conversation to the detriment of everyone at the table. It's a group activity and needs to be treated as such.
There's not much you can do about a DM hogging the spotlight.
When it comes to spotlight hogging by players, though, I'm a bit more law-of-the-jungle than some: if you want the spotlight, try doing something proactive to get it rather than just passively waiting for it to come to you.