When does discounted first show up?
 

How far in advance do they start to sell these? I'm planning a trip for April 7(SEA-FLL-SEA) April 15, and I'm wondering if the fact that there are no discounted first seats showing means that there won't be?

Not necessary true. CO revenue management tends to lower FC fares - when they believe they will have many unsold seats. FC fluctuates. So if you buy ahead - you might pay more - just like coach. But then it is a gamble. Best thing to do is to watch the seat maps - if none to only a couple of seats are sold - say a week or two before - or even longer if a true vacation destination - then you might see prices drop.

Don't forget that a lot of discounted F/BF fares go away 50 days before the date of the flight. You really need to check out the fare rules for details of what is published on the route you wish to fly.

I'm confused, because KVS shows Z9 for all of the flights, but when I try to book it only shows 'A' fares.

Does CO publish a Z fare between FLL and SEA? If so, are you complying with the advance purchase and minimum stay requirements?

It shows up on KVS Availability Tool, and actually I think it's the fare I usually get. How can I see what the fare rules are?

Discounted domestic FC fares are in R-class are they not?

Entirely irrelevant.

Take a look at Question Number 1 in the KVS Tool FAQ :) -- there are no Z-class fares published for your route:

[KVS Availability Tool 2.8.0/Platinum - Sabre: Fares/ZUJ/SG-USD]