Here is the important link, which 95% of the time will let players resolve rules questions on their own:
https://www.swmgamers.com/SWM/Rules/ResolvingEffects.htmlThat link is also key to this ruling, but it is a little bit tricky.
When reduced to 40hp Calo Nord BH declared Presumed Dead, which interrupts the Resolving Effects series that dealt that original damage. Immediately before resolving Presumed Dead he declares Last Stand and uses Grenades 40. If the target enemy is adjacent, Calo is subject to the Grenades 40. What happens if he fails Grenades, Beskar, and Avoid Defeat in succession?
In this situation, Calo would be actually dead if he fails Grenades, Beskar, and Avoid Defeat in succession. The Resolving Effects sequence of the original damage is suspended, and the resolution of Presumed Dead has not yet begun. Both waiting for resolution while Grenades 40 is resolved. If Calo is defeated during Grenades 40, that defeat is resolved completely (Calo is removed from grid), and at that point it's too late to finish resolving Presumed Dead. Presumed Dead cannot be declared on the Grenades 40 itself because that would be the 2nd time Calo's hit points are below 40.
This is the wording that is in Tactics Broker: "Remove that character from play; that character is defeated." That wording prevents the use of Avoid Defeat or any other shenanigans. Removed from play happens in step 14 of Resolving Effects, and is the same thing that is described in Tactics Broker. That is why all of the defeat-prevention happens in step 12 - before anyone is removed from the board.
As a separate situation/illustration, if the original damage was a 50dmg hit that took Calo all the way down to 0hp instead of just to 40hp, then Presumed Dead would trigger in step 12 to prevent defeat during that Resolving Effects sequence. But he has to get his HP back above 0hp before step 14 (and Presumed Dead would do that for him). But if a new Grenades 40 Resolving Effects sequence is triggered and he takes damage in step 12, then you check for defeat and once again he "would be defeated". (Side note: if he doesn't take damage, you don't check for defeat even though he's at 0hp... in that case Presumed Dead can proceed as normal.) If he "would would defeated" then he'll be defeated for real in step 14 if he still has 0hp. Note that in this situation, if he fails Grenades, Avoid Defeat could still save him but Beskar'gam would not. If he survives the Resolving Effects sequence for Grenades, then the declared Presumed Dead would be resolved.