Let me try to help and clarify.

First, CX had retained 845 leading up to 4/5 originally intended to be the last flight out, but subsequently cancelled flights down to 1 ex-JFK in first week of Apr.

The timing of flights is being modified because CX is unable to take off after 8p from T8 - T8 only has 1-2 transcons left these days and my guess is TSA wants to close early - hence CX would need to either delay until morning, as in 3/26 845, swap to 831/841 as in 3/29 831, or take off from another terminal that operates (T4). CX prefers the former two options as they can afford to park a 777 frame overnight, off the terminal, with no need to get it back to do intra-Asia / EU run.

I can say that these days flights are mostly full but not necessary at 100%+ across all cabins. J is being overbooked with pax downgraded. You will also have a lot of non-duty crew flying occupying a whole row (A, B, C) when possible. Being the last flight out, it is going to be a crowded flight from cancellations and consolidation.

As a side, LHR loads have been very low.

By the way, most pax are well prepared now with protective equipment, hence hope OP's father is as well, and wish him a safe journey.