I can't answer 90% of your questions, as I'm using my Deneb on an A4000 with OS3.9, but I wouldn't sell my Deneb card for the world.
I've installed the latest BIOS update (a simple matter of downloading a file and following the instrcutions) and it works in perfect harmony with my Cybervision 64/3D and FastATA4000.
I keep a USB Ethernet device plugged into the internal port at all times to put my Amiga online, I have a USB to PATA/SATA cable for hooking up stock 3.5" and 2.5" drives directly, I have a USB PC floppy drive (doesn't do Amiga formatted disks, but I have an internal HDFDD for that) and a bunch of memory stick that I use.
I had been fiddling with a USB to SCSI device before I left home, but I haven't finished testing that yet.
Using it is easy. Once you have Poseidon installed you can access it through the Trident interface in Prefs. Everything mounts automatically: Just plug in that USB cable and a Poseidon window pops up to tell you what it has detected. Unplug the device and another window pops up to wave it goodbye. The USB stack even updates itself over your internet link. Brilliant!
