And when the Torah was revealed by Moshe Rabbeinu, the previously hidden splendor wasn’t at least as great?
Your analogy is not valid.
We were commanded by God to eat matzah and marror on Pesach, while we were not commanded to do so on Shabbos.
OTOH, building bonfires is not a command from God, but something that we, ourselves originated. As such, if we can do so for Lag B’Omer, I don’t see a reason why we can’t do so for Shavuous as well.
1: No. Many “Frum Jews” do not listen to the 5 books of Moshe, and Nach certainly not, but no “Frum Jew” would even think of transgressing the Kabbalah (Zohar) or the Tzava of R’ Yehudah HaChassid.
2: They believe that God did command them (via the Rebbes) to build bonfires.