As far as I can see, the only ones available on the Brighton side* are in the second tier - these are "Club Wembley" tickets which are held as debentures and can legally be reallocated (/resold?), similar to 1901 seats at the Amex.
*apart from a single listing which doesn't say where it is...
The ticket allocation to Friends/Family is exactly the same as at the Amex (because it's using the same website).
I'd suggest calling supporter services - they **might** be able to reallocate your tickets to get them together, but I wouldn't hold your breath (especially as they're not open...
https://www.brightonandhovealbion.com/news/3118110/emirates-fa-cup-semi-final-ticket-information
No date/time yet, but confirmation of 34,000 tickets and allocation by points in essentially the same way as other Away fixtures.
That was a weird Covid-impacted one, though, with only 4,000 in attendance. The scheduling was probably done for TV.
The 2020 matches were at 19:45 on Saturday and 18:00 on Sunday, with no attendees.