Does the reward for Jumu'ah (Friday prayer) disappear completely for whoever does not enter the mosque before the Khatib ascends the minbar, or does the reward disappear only for whoever engages in idle talk during it?

Hastening to Friday prayer is desirable. The angels sit at the doors of the mosque, recording the names of those who come early, until the Imam emerges, whereupon they close their scrolls. This does not mean that whoever arrives late, after the Imam has emerged, misses the complete reward of Friday prayer. Rather, they only miss the reward for early arrival and promptness. As for the reward for listening to the sermon, praying, engaging in remembrance, supplicating, and observing humility, this is recorded by the two guardian angels entrusted with recording good deeds.