There are clues in Scalegorge Waterscape which indicate Dan Feng secretly snuck Yinxing into the sacred grounds more than once to do some kind of research.

Any hint on the time frame? Has he been doing this before the Battle of Shuhu or only started afterwards?

for people who don't know the original Quintet, they wouldn't automatically make the connection

Does this apply for the titles of the Quintet involved? Since Jingliu mentioned both Baiheng's name and her title and Yanqing was still like "uuuuuuh...."

In total, the Astral Express spent a few months on the Luofu before heading to Penacony. The protagonist spent several weeks helping rebuild the economy of Aurum Alley, and then some time helping Guinaifen's team with their ghost hunt.

Huh...ok, I was under the impression that the team basically went off, since the end of 1.3 was basically Jing Yuan bidding farewell. But ok.

I still haven't found anything that clarifies Jing Yuan is the youngest of the Quintet

There's no canon support for this.

OK. Good to know. Then I will stick to my personal headcanon that Jing Yuan was actually an adult during the era of the Quintet.