I like the third suggestion (remove wall upon certain damage to boss). This seems like the only skill-based solution here.
Ammo at the wall seems unnecessary with changes to melee damage vs. monsters and rewards you for bad resource management. Same for the increase to monster damage as last man standing. The change to melee damage already helps compensate for this.
The respawn at the wall is an okay solution but feels a bit too easy. It sort of begs the question of why your teammates are dead in the first place and removes the penalty for playing badly. If we’re being honest, you probably weren’t going to clutch revive your team with a boss on you and rushing into an area filled with new ambients anyway - unless you were a high-mobility class (Handmaiden). Hell, you’re probably going to struggle to revive at the wall with a boss if you’re alone – unless, again, you have high mobility or are able to manipulate terrain/monster pathing. If you have two people, then just kill the boss…
To be honest, I’ve rarely seen this as a problem on my recent runs. The bosses go down super fast with the changes to melee damage and are now pretty easy to solo. So /shrug?