Well there are many ways to fix the problems you come up with.
1, the game could check to see how many players are near the boss when it spawns or the fight starts
2, Monsters wouldn't change hp. They would spawn with it. so say there are 2 players on and a spider spawns with 50hp. A new player joins making the player count go up to 3. The spider would still have 50hp but any new spiders that spawn would have 75hp
I don't think normal monsters should have more health anyway because most of the time its 1v1
I see what you mean but they could just make it so you set the health of bosses. much like you can set the spawn rate of thing now.
I feel like the normal monsters health is fine how it id now. Its the bosses! Having a options to set the health would be great and open the game up to so many people.