Part of the stickiness of this situation is that we can't adjudicate the future. The future where Sorin was played properly never arrived. But let's break down scenarios and see if they provide clarity.
1) Player interrupts self.
I announce
Sorin, Grim Nemesis but reveal
Sorin, Lord of Innistrad and stop myself.
I play
Sorin, Grim Nemesis and realize I only have 5W available and stop myself.
We've long held the idea that players may interrupt themselves while taking and action. In these scenarios, we allow the player to stop their current action, rewind associated actions, and restart or abandon the action. These are unintentional actions.
2) Player makes a legal but unintended play.
I play a Swamp when i meant to play a Plains without announcing anything.
Though the action may be unintentional, we have no countervailing action to indicate an issue. If the player is still in the process of doing the thing (hand on the Swamp, for example) and notice the error, then they are interrupting themselves. If their hand is off the card or the game has otherwise moved on, then the action is as it stands. The takeaway lesson from this is simple: use words
and actions to do things. When words and actions don't line up, then we can room to resolve the discrepancy.
3) Player makes a legal, deadly play.
I announce
Sorin, Grim Nemesis while at 1 life with my only W source being
Caves of Koilos.
Sometimes mistakes are more costly than others. At competitive, players sometimes pay dearly for making mistakes. A smart opponent will quietly wait for their opponent to pass priority or some other action to indicate they are moving on before reminding them they're dead. If an opponent speaks up during a player taking an action, then that player may realize their mistake and interrupt themselves. It is sporting for an opponent to speak up an cause an interruption, but it is not unsporting to quietly wait for death before saying something. Such is the nature of competition. (At Regular REL, this is a good chance to educate players and keep the game moving along.)
4) Player makes a legal play. looks for a response, and then makes the play illegal.
I announce a targeted black spell without specifying a target, look at my opponent for a reaction, and then choose an illegal target.
Fishing for responses is not allowed. There are two factors that make these cases obvious: pause for response and intentional choices to make something illegal. If a player announces an illegal thing without hesitation, they have violated a GRV, but they haven't demonstrated an illegal intent. Mistakes happen. But when a player injects time and observation into the mix while waiting to finish an action, they've telegraphed their willingness to intentionally break rules, and when they then do break rules, it's cheating.
Consider the following. The opponent has only two creatures.
Player:
Throttle your
Insidious Mist.
Opponent:
Merciless Resolve my, wait, what?
Player:
Throttle…
Opponent:
Merciless Resolve my zombie.
Player: Target your
Insidious Mist. Guess we have to reset everything. Go.
The pause (fish) and choice (illegal) make cheating obvious.
5) What we've got here.
The player in the original post may or may not have intended to bait their opponent. And the opponent should have waited. But as written it's clear to me that the opponent was actually being nice and functionally saying, “Look, Sorin is not going to help you. So think about what you want untapped.” Because of the fidgeting, I don't think the opponent was trying to force an interruption to then hope the player tries to intentionally make an illegal announcement and thereby intentionally break a rule. That is too meta for my tastes.
What is clear is that while the player was fidgeting, the opponent gave away more information. The player then chose to take actions (non-actions, actually) to force an illegal state and gain an advantage (bonus information and no countered spell). Do I think playing Sorin or fidgeting with the lands was baiting? Maybe, or maybe not. But I don't think that that actually matters.
If the player said, “Ok, well, if I can't have Sorin, I'll leave these untapped,” no one would have any issue. By choosing to make his announcement illegal, he's getting advantage out of the illegality of the situation. I would likely DQ the player. The only scenarios in which I might not would have to involve the player pointing out that he was in the process of trying not to kill himself with
Caves of Koilos or perhaps have specific mana available after playing the spell. But in a Swamps and Plains world, the answer is black and white. The opponent's actions introduced the possibility of a cheat of opportunity, and the player took that opportunity and cheated.