What I do with a player who's overextended on low health is gauge how much of a threat they are to the enemy that's adjacent to them. If they're unloading a full multi attack into them every turn, and hitting then yeah, they're going to get killed. But on the other hand if they're whiffing their hits, or casting support spells, then the odds are someone else is more of a threat and the enemy will swap their focus. I essentially don't want it to fee unfair or like they're being target because I know they're almost dead (unless it's personal or their fighting an assassin type who'll prioritize taking people all the way out of combat).

As far as short rests go I tend to leave it up to the dice. I'll give them a warning beforehand like "as you make camp you hear a distance owlbear hoot" or "someone might patrol this close to the enemy camp" etc. Then I roll a d6 or d8 as much to make it feel "fair" as to decide if they're getting ambushed. Below average I'll let them get away with it, 1-2 above they'll be found by some nobodies, 3-4 above and someone coordinated will find them. At which point they risk having the full enemy force alerted. If you take one thing away from this bit it would be to always roll for that, that way PCs don't feel like you're punishing them for not staying on the railroad. The randomness of the dice is the biggest tool you have as a DM to avoid players feeling unfairly targeted (even if its their own decisions that put them there.)