But if they are rules, then (whether or not everyone was on board for them), they must be applied consistently. That doesn't mean they trigger every second. But when they do, they should be used.
And no, things do change. Expectations are often not met. Conventions can be set aside at any time for nearly any reason. Rules, in order to be "rules", must have SOMETHING to them that actually makes them normative. If they are not, in fact, actually normative--if they are simply guidance or a gesture at something or whatever--then they are not rules.
That's what "rules" means. Without actual force, they are mere suggestions.
If it has no published rulebook and 100% of the things as part of the game are purely conventions, it has no rules.
"Poker", to give you an example, has almost no rules because without greater specificity you know almost nothing about it. You don't know whether it's five cards or seven cards or more or fewer. You don't know whether there are community cards or not. You don't know the scoring, etc., etc. You don't even know if the dealer is a player or not. Without greater specificity, just about the only "rules" are that that players get dealt some number of cards, and that rarer combinations of cards have greater value, but you don't even know the specific combinations.
Absolutely the hell not, considering we just had a recent thread about this stuff and the majority opinion was "roll in the open."
It matters, because a rule has normative force. A convention does not. "Maps are oriented with North pointing up" is a convention, vastly more popular than any alternative. Yet nothing even remotely stops a person from publishing a map with any direction, even an ordinal direction, pointing up. Some maps even can't be oriented that way, e.g. polar maps or ones that attempt to preserve area without preserving direction (the "orange peel" type maps).
Popularity is irrelevant to whether it has normative force. Plenty of laws are quite unpopular. That doesn't mean it's suddenly not a law anymore.
Just like laws, rules are rules because they have normative force. If someone actually breaks a rule, their behavior has to be corrected. If a statement can be ignored or not at leisure, it has no normative force. It isn't a rule. It's a guideline, suggestion, recommendation, or piece of advice--not a rule. This applies as much to house rules as it does to anything else; a house rule is not a house suggestion, it is in fact actually a rule that people are expected to follow.
Even DMs have rules--even in 5e, as much as it tries to position the DM as an autocrat who does whatever she wants, whenever she wants, for as long as she wants, purely because she wants to do so, for any reason or no reason at all.
No. It's when it has actual normative force. While "expectation" (dominant or otherwise) is technically irrelevant, it is important that the rules actually be communicated to the player, which might qualify for however you define "the expectation." Something that has normative force, but which is concealed from the people upon whom that normative force applies, is not just a rule--it is coercion, or worse.
Simply put: Does it have normative force, or not? If it is in fact normative, then it is a rule. If it is not normative, if it is merely suggestive, allusive, advising, recommending, etc., then it is not a rule.
Whether you choose to call it a rule does not actually affect whether it is a rule. People call things by incorrect names all the time.
Fudging is when a person (generally, the DM) lies about some result or figure, generally a die roll, claiming that that result was something other than what it actually was. I say "generally, the DM" because when players do this, it is called what it is: cheating.
Fudging cannot be a rule because, by definition, it is lying about the results. Hence, it is (by definition) breaking the rules. I'm aware that the text of 5e has a non-normative suggestion that DMs should lie about results if they think their false result is superior to the true one. I strenuously disagree with this suggestion, and find it both patronizing and insulting to the players subjected to its "advice."
When it has normative force. It is a very simple standard. As soon as something actually has normative force, it is a rule. It doesn't matter whether this normative force was acquired because a designer wrote it down, or because an old game fell out of fashion and a new one replaced it. Chess, as we play it today, is rather a different game from its High Medieval counterpart; what we call "chess" today was originally, yes, a convention called "Mad Queens" chess. It was, in fact, a scandalous SJW game in its heyday, because women could play chess just as well as men could, and could beat them, and (worst of all!) a woman was the most powerful piece on the board, while a man--the king, for God's sake!!!--was the second-weakest piece on the board.
It really isn't. A rule has normative force. Non-rules do not have normative force.