War Caster lets you cast a spell instead of an opportunity attack, which is made just before the creature leaves your reach. If your reach is 5 feet and you use War Caster to cast a ranged spell attack then, yes, the attack is made with disadvantage.
If you have a natural reach greater than 5 feet, then the attack is made when the creature is more than 5 feet away, so this doesn't apply. Similarly, it doesn't apply to melee spell attacks or spells that use saving throws. You are also good if you have the Crossbow Expert feat.
Clever, but unlikely
In order for Catapult to work, it requires:
one object weighing 1 to 5 pounds within range that isn’t being worn or carried.
This is the requirement that Delayed Blast Fireball must meet. The question is: Does it?
Delayed Blast Fireball isn't explicit in the definition, but it does state:
A beam of yellow light flashes from your pointing finger, then condenses to linger at a chosen point within range as a glowing bead for the duration.
How much does a bead of light weigh?
In order for that bead to qualify for Catapult, it must be at least one pound and less than five pounds. As Dale M states, that's likely a DM call. But how should the DM approach?
Well, it's a beam of light that condenses into a bead. How much does a bead of light weigh? I'd think that a bead of magic light does not weigh anything, but another DM may believe otherwise. Then it's a question of does a bead of light weighs more than 1lb. Even if it did have a mass, it's unlikely to weigh more than 1lb.
Without meeting the weight requirement, you can't use Catapult to launch the Bead for extra damage (pending the save failure).
Throwing the Bead
There is another option for interacting with the Delayed Blast Fireball:
If the glowing bead is touched before the interval has expired, the creature touching it must make a Dexterity saving throw. On a failed save, the spell ends immediately, causing the bead to erupt in flame. On a successful save, the creature can throw the bead up to 40 feet.
This does imply that it can be handled, but it doesn't answer weight or anything else. Ultimately, this is a magic bead made of condensed light and it does not necessarily mean it has mass. It just means that you can throw the magic bead. If a DM does rule that it has mass, then it still needs to weigh more than 1 lb (see discussion above on weight of condensed light.)
Best Answer
No, Seeking Spell does not work with fireball.
Seeking Spell states:
If we look at Fireball, we see:
Fireball does not have a target line of one or more creatures, nor does it require the caster to make a ranged touch attack. There are just instances where you need to do so.
An example of a spell where you're required to make a ranged touch attack is Scorching Ray, which states:
A tangent on the usage of Seeking Spell.
If we look at the rules for magic we see:
Seeking spell allows us to bypass the requirement of having a straight unblocked path for casting a spell.
This means that spells like Magic Missile or Scorching Ray, could be cast if you have line of sight to a target, but not line of effect. It also allows you to ignore Cover and Concealment with spells like Searing light or Scorching Ray, helping to increase your odds of hitting a creature.