yes, the instances of forking are about it being legally fine. nothing to do with the psychological overwhelming of forks.
also, death of the author just means that the interpretation of a work is independent from the author's intention. nothing to do with categorisation.
I think it addresses that by explaining that forks rarely become many.
yes, because people add plugin APIs/modification APIs. which makes making many forks unnecessary. even the Linux kernel has a modification API; they're called kernel modules.