It sometimes happens that several identical monsters, all using the same initiative, attack a single character.
In my games, this is often a volley of missile fire after a PC has shown themselves to be a caster.
For practicality, I typically roll all the attack rolls together, count up the hits, and then roll all the damage, and assign it to the player as a single total. If the potential damage total is less than their current hit points, and the PCs have no relevant reactions available that might interrupt or affect the attacks, there is little difference between this approach and RAW.
However, when there is a potential for damage to render a character unconscious, this approach does differ substantially from RAW. RAW, I should roll each attack and record each instance of damage separately. Upon the first hit that renders the character unconscious, they immediately drop prone. The subsequent attacks, although occurring on the same initiative, in some sense come 'after' the character has fallen prone. Thus they are at disadvantage to hit (assuming missile attacks) but each one that does hit indicates a failed death save, so that three such hits would result in the character's death.
Is my understanding of the situation with RAW correct?
Is there any difference in this situation between multiple attackers on the same initiative and an attacker with multiattack?
Suppose I choose to roll all attacks and damage at once, even in situations where the potential damage was more than a PC's current hp. [In this case, the chance of the attackers hitting would increase, the chance of the PC going unconscious would increase, but the chance of the PC dying from failed death saves would decrease.] Can this decision be reconciled with the rule (Initiative; PHB, p. 189):
If a tie occurs, the DM decides the order among tied DM-controlled creatures
That is, have I decided to resolve these ties simultaneously? Or would such a choice violate RAW and require me to invoke Rule 0?
This question is primarily about the mechanics of event timing (multiple attacks, unconsciousness, dropping prone) within a simultaneous initiative. It is not about DM practice, what I "should" do, the broader consequences of using such a procedure, when such a procedure would not apply, or what PC abilities exist that might affect the attacks.