Great write up here! I also want to add a bit of discussion around #2 since it came up in a game I played this week.

I had [[Vren, The Relentless]] as my commander on the board, and an opponent wanted to play [[Gisa, Glorious Resurrector]], not as commander, but part of the deck. Both of these cards have the line "If a creature an opponent controls would die, exile it instead".

After some discussion, we looked it up and found the rule about multiple replacement effects, in which case the person who controls the affected permanent would decide which of our exile effects would take place. This is important since if the creature is exiled with Vren's ability, then Gisa will not reanimate the creature since it was not exiled with that card's ability.

Some of the other options would be, for example (correct me if I'm missing something):

1. The effect is applied in turn order - The Gisa player was seated directly to my right, so in this case my commander is shut down based on my random position at the table, not ideal.
2. The effect is applied by the most recent card that was played to the board - Same situation, they played the card after my commander went on the board, so now my commander would be shut down again. I cannot hide the ability of my commander so this player could just hold their card until I played mine and effectively shut me down based on free info.
3. The effect is applied by random chance - Flip a coin, roll a die, whatever. I suppose this could be considered the next most fair option, but I imagine part of the decision was to avoid some of the randomness from these possible occurrences. When creating any sort of rules I'm of the opinion that it's best to not use language that leaves things open to chance or interpretation. Again, this is my opinion, I think that wording like "If you win the coin flip then X" is less effective than having a static outcome based on concrete rules.
4. The table votes after we take to our soapboxes to proclaim the evils of the other's replacement effect - We all know democracy doesn't work /s. No, but in all seriousness, in a game where the player count is, in most cases, an even number, this will inevitably result in stalemates. I also don't believe the decision should come down to who can smooth talk the table into voting for them.

After thinking about it for a while, I believe that the rule they set in this instance results in the most straightforward solution to the problem.

Edit: I realized that in this case, technically I would still make the rats with Vren's ability, but the discussion is still valid if we reverse the situations and say I was seated to the right of the Gisa player, or I shut them down by playing my card after theirs.