You write on your website:

"(b) The reason the third of Tishrei needs Chizuk is not because we are afraid that people will not celebrate that day. Rather, it needs Chizuk so that people will remember the miracle for which the new Yom Tov was made. Even though the observance of the Yom Tov of the third of Tishrei does not need Chizuk because it is a Yom Tov m'Divrei Kabalah, it nevertheless needs Chizuk so that people will remember the miracle that happened for which the Rabanan made a new Yom Tov."

I think this answer needs clarification. If the purpose of making a yomtov derabonon is to have people remember the miracle, then why would the gemora ask "what's the point of making the yomtov when there already is a yomtov". The answer should be "so that people will remember the additional miracle".

Implicit in the gemorah's line of questioning is that there needs to be a practical result of instituting a new yomtov. In this case the practical result is that the day before becomes assur btaanis. But that's only true because in general the rule is that a yomtov drabonon assurs the day before it. In other words it's a lo plug - which is what the Pnei Yehoshua said in the first answer you quoted.

Of course the motivation of the rabonon was to commemorate the additional miracle, as you say, but they couldn't do this without a practical result. So they took advantage of the fact that the day before would become assur (as it always does), even though it wasn't necessary from a chizuk standpoint in this case (because it's already a yomtov midivrei kabala).

I think the 2 answers you gave complement each other in this way.

Dov Silverstein, Passaic, NJ USA