Let me write down the supporting theories I know of for each character and add on my comments:
Yukari - Strong evidence suggests that she wrote the vampire contract.
She is involved in many of Gensokyo's activities, including but not limited to Lunar War, barrier of illusion and reality, creation of Hakurei barrier.
Therefore it can be safe to assume that she could possibly have suggested the Spell Card Rules.
Holding back is something mentioned in Yukari's profile, which is concept 3.
Con - Does not know Reimu prior to her introduction.
Does not have a beauty theme.
Yuuka - Her theme fits the Spell Card Rules. PoFV, which is her first Windows appearance suggests that she values beauty.
It can been seen in her motif and what she does.
Reimu knows her prior to her introduction.
Her profile suggests she fights with certain rule sets, which can possibly be what spell card rules become today.
Con - One of the spell card rules listed attacks of physical attacks are not to be repeated, this is not what Yuuka would like to propose.
@Spell Card Rules:
Yes, you are right that it never said those same youkai also suggested the rules. However, the implication is there.
It is hard to believe that those same youkai would not have inputted their idea on what kind of rules they would use to make life less boring.
It is only logical to believe that those same youkai who had a discussion with Reimu would also be the ones that created a draft of some kind.
As for the amount of youkai in discussion with Reimu, there are probably not a lot of them. Unless there is a translation error, it seems to imply that only the more willful youkai are in this discussion, so my assumption is small number.
These are the relevant texts from PMiSS with suggestions on creation of Spell Card Rules:
Some of the more willful youkai found these terms distasteful and entered into discussions with the Hakurei shrine maiden.
The shrine maiden, who was also tiring of an uneventful lifestyle, agreed to the youkai point of view that some battles were necessary.
The result of this were the spell card rules, which allowed duelists to seemingly stake their lives in individual combat (*4).
4: It appears various other rules were also considered, but as the beauty and volume of danmaku especially appealed to youkai and women, they aren't as popular.
On the Draft of the Spell Card Rules, PMiSS suggests that a youkai proposed it to Reimu. Since Akyuu wasn't actually there, she can only have theories on who actually proposed the draft.
She's almost never shown in the series after PoFV and she doesn't really do much of anything. The formal battling fits, but her personality and style is quite the opposite of what's described in the rules. Yuuka isn't interested in "incidents", she never goes easy, likes to use physical attacks while neglecting her own magic and ability, and is only interested in powerful opponents rather than evening the field between all fighters.
Wait, I am confused, I don't remember it ever saying that she never goes easy. I don't believe there is any indication of that matter.
Is there a passage somewhere that implies that this is the case?
She does like to use physical attacks and is interested in powerful opponents.
I was under the impression that the first 2 concepts were that youkai did not want to live a boring life style. The 3rd concept is precaution, because going all out has repercussions on Gensokyo.
I did not mention the first 3 concepts, because they would fit any youkai that would propose this. The 4th concept is the only one I mentioned, because that concept is more specific to some youkai.
