Stephen writes
Do these plants die down then and have to be regrown from scratch, or
do they continue to grow into a small bush or tree?

They are perennial, but because they are not native to the UK, they
don't like the low light levels in winter (and certainly wouldn't
survive outdoors). So although you can keep them over winter, they get
leggy, and die back a bit, and it's generally better to re-grow from
seed each year.

I'll remember about the pollination, at least I will have to, because I
haven't had two flowers at the same time yet. Unless it is possible to
pollinate with one flower?

Yes.

--
Kay