Please note that the "set" rule has nothing to do with this scenario. Since the word "set" is not used in the text of either plot, it does not apply here. (Putting the word "set" in quotation marks in the RRG entry you quote indicates that the word is what invokes the rule, not the act of determining a lasting value for a particular variable.) Since you do not have "multiple set modifiers," the rules regarding a conflict in multiple set modifiers does not provide your answer.
What does provide your answer is "Priority of Simultaneous Resolution." But first, let's make sure we are all starting at the same point.
- Summer Harvest initiates and Player B chooses Player A, whose plot card has a printed income of 5. So the "when revealed" on Summer Harvest creates a lasting effect (it creates an "effect or condition that affect the game state for a specified duration" - that specified duration being "while the affected card is in play" since no particular time period is stated - so yes, it is a lasting effect) that makes the "X" for its income "X = 5 + 2 = 7."
- Varys' Riddle then initiates, and Player A chooses to initiate the "when revealed" on Summer Harvest, choosing Player B. Player B's revealed plot card has a printed income value of X, which is treated as 0 for calculations as per the RRG, thus creating a lasting effect that makes the income value of "X" on Summer Harvest "X = X + 2 = 0 + 2 = 2."
Ultimately, the First Player gets to choose the order in which the "when revealed" effects initiate, but since they both create lasting effects (not "set" effect), you have are really dealing with competing, simultaneously valid lasting effects - one that makes X = 7, the other that makes X = 2 - when the Summer Harvest player counts income. According to the RRG entry for the Priority of Simultaneous Resolution:
"If two or more constant abilities and/or lasting effects cannot be applied simultaneously, the first player determines the order in which the constant abilities are applied."
So, the first player gets to choose the order in which the "X" value lasting effects are applied (with the last one applied controlling) when the value of X is checked for the purpose of counting income (or anything else that references the (non-printed) income value on Summer Harvest).
In terms of your bet, you're not correct in that choosing the order of the plot resolution does not dictate the ultimate value of X. But you are correct in that once all the "When Revealed" effects are resolved, the First Player will have control over whether X is treated as 7 or 2 when a (non-printed) income value is referenced for Summer Harvest during the round.