There should be an MI version. The Archimedes parallel port is a very simple device and should be easy to support. The A3000 TRM explains how it works. The original Archimedes hard disc interface is based on the Hitachi HD63463. Linux has a driver for it.
Support is in the source tree, but is mostly untested.
This should be fixed. The Castle EtherSCSI card contains an AMD 53c94 and an Intel 82595. The Storm16 seems to be the same card without the Ethernet components fitted. Nexus was SJ Research's proprietary ATM network system. All the higher-level protocol is handled in software, but implementing PPP over it ought to be easy enough.