So basically other than the riptides and a single AV 10 flyer you don't have much to stop even Hive Tyrants much less multiple Nurgle daemons with 2+ jink save. Or perhaps a Screamerstar, the list isn't bad at all but what to do when you run up against a particularly resilient enemy?
You're throwing a very specific example out to counter this list; I know Tau is usually heavy on anti-air, but how many armies do you know that's carrying that many anti-air units for FMCs? Two Riptides, a flier, and massive quantity of autocannons rerolling their snapshots SHOULD handle anything. In your Monster mash example, each dead Demon is worth a good chunk of their army.
As it stands even a mounted Seer Council can not only reach you quickly, but dismantle your army piece by piece.
I would consider giving the Knight 2 Bright lances, and the sword+sheild and give the Farseer the Wraithforge gem, that way you can at least tie up super units like the Screamer star and heal off wounds allowing you to dismantle the rest of the list.
Again, you seem to be focusing on what a Demons of Chaos list will do... have you had bad experiences with them? I like to build my list for all comers, and it feels foolish to kit out the Wraithknight for close combat when nothing else in the army is built for it, just in case I get a death star coming at me. Besides, in a situation like that I can do the same thing that you suggested against me; overwhelming firepower. Each squad of Broadsides can unleash 18 shots a turn, and I have 4 of those squads.
I would also look at getting something other than just a 2+ armor save or a 4+ cover save, especially since a enemy Riptide could instakill a squad of broadsides each turn PER riptide. A landing pad or some void sheilds would go a long way to keeping you alive.
Really not interested in Fortifications. Anything a Riptide can do to me, I can do to it.
I don't know anything about formations though, Tank hunters on all that Str 7 sounds amazing, what book is that from.
The Advent Calender Dataslates that GW sold. They had a few doozies of Formations, including the Firebase Support Cadre and the Astartes Storm Wing.
Edit: Because you have all that Tank Hunters you don't need a Fusion blaster on those Riptides, a SMS would be better for people cowering out of LOS like enemy Jetbikers.
The Fusion Blasters on the Riptides are more there because I might get a Heldrake or Stormraven flying in, and I want to pop S12 armor on a flier. I have 12 SMS already on all the broadsides to deal with cover if needed, and they don't synch very well with the Ion Accelerators.