In general, a player must not interrupt the DM to check the operation of the rules. What the DM says is final, and if the players do not agree with the DM's handling of the rules, they must wait until the end of the session to resolve their disagreements.
So with this player, you have to be very clear and direct by telling them that even if they're absolutely right, they have to wait until the end of the session to discuss errors or changes in the rules. Then, based on this discussion, you can agree to replay certain events from the previous session, or follow different rules from the front, or you can state that you will follow your understanding of the rules instead.
As a DM, you have a responsibility to understand the rules. Players have specific class characteristics, actions they can perform during a fight, spells they can cast, etc., and so on. If you do not follow the rules to the best of your ability, or if you do not follow the rules as they are, you deliberately write, it will be more difficult for players to make informed decisions about how to play. their characters.
This is not necessarily limited to changes to rules or homebrew hardware: an assistant will probably keep Identify in their spell book to help identify the magic items they acquire, and if you use the variant rules for identifying magic items (DMG, 136), Identify Spell alone is not enough to completely reveal the properties of a magic object, it could get upset if you do not inform it that you plan to use these rules at the same time. In advance because it is a decision to create a character taken under false premises. . Now, it's a relatively minor and manageable example in most tables ("All right, I'll let you trade that spell from your spell book with a different 1st level magician spell for free") but larger gaps could be more frustrating for players, because if the rules are not well defined, players will feel that they can not make meaningful decisions.
Compared to the above, the introduction of new mechanisms regardless of how they interact with the world into which they were introduced can have negative consequences.
An example that appeared in a recent campaign that I participated in was the decision of my DM to add a new type of creature to the game to represent the central villains of the campaign. I think the intentions of the DM were to create a villain who would have been foreign to Faerun's decor and who would probably have succeeded. But then the problem became that this new type of creature had no interaction with the abilities or features that any other character had:
- It was a NEWTYPE, so the spells that say "affect Aberrations, Celestials, Fey, Fiends, Undead and Elementals" will not affect them.
- Their special abilities were NEWTYPE, for example, an aura of the Paladin that says "gaining spell damage resistance" will not affect their abilities "magic type"
- It also meant that their features bypassed elements such as anti-magic fields.
- Their type of damage was NEWTYPE, which means that no feature that confers damage resistance (or vulnerability!) Can interact with it, unless the message "Resistance to ALL damage" is read, which is a very small subset of features.
As a result, it was impossible, as players, to make meaningful decisions regarding the preparation of battles with this type of creature. We could not revise the class characteristics without staying home, and we could not prepare spells to deal with these creatures because most of our spells would not work at all.
Eventually, we were able to convince the DM to incorporate NEWTYPE creatures into "Aberrations, Celestials, etc.". lists that lots of spells have, which allowed us to make tactical decisions when you approach such encounters.
Also, in general, it recalls in a dramatic way the frequency with which they appear in the meetings; we have participated in over a dozen sessions since the last time we fought a creature of this type.
As a DM, I spend a lot of time trying to look at extreme case scenarios in D & D to better understand rule interactions. Sometimes, if I see something odd, such as a long – range archer who improves his chances of attack by going blind, I expressly decide that not following the rules such as the they are written is necessary to improve the game.
But here's the trick: Even though I do not intend to follow the rules as they are written in a specific scenario, it is still important that I understand what they are. areand why they were written that way, because it helps me better understand the problem I'm trying to solve and better justify my decision not to follow that rule.
So, since these disagreements on the rules affect not only your relationship with this player, but also your relationships with other players, it is important to make sure you understand the rules and develop a good rationale for not following them. that & # 39; written. Planning meetings where you do not know what types of mechanisms might interact with them will eventually be frustrating, even if players know better than to express that frustration in real time at the table.
D & D has always had its roots in the game mechanics of Wargaming. So, in principle, a player "playing to win" is not a strictly invalid playing method. D & D, even the 5th edition, facilitates this kind of metagamey, "here's how we develop our strategy to win!" the gameplay, and many groups that do not play this way often do so by evading the mechanics of the game which, in their opinion, do not contribute to the desired style of play.
However, the evidence strongly suggests that this is not the kind of game you want to play. So, what's very important is to sit down with your players and decide what kind of game you want to play. If you all agree to play a game of Wargame type or if you agree to play a game more social and less focused on the fight, then everything will be fine. But if you try to play different games, you will not have any more problems.
Many problems at the table can be solved by discussing openly and honestly what is not working and what needs to change.
I've insisted just as much on involvement, but if you're ready to talk to your players about what frustrates you about the gameplay, and what they're ready to listen to, you'll find a solution which will suit everyone. .