The swarm is not affected by spells & items that affect only a single creature, so your buffing options are limited. The "pernicious poison" spell would help, but even then a scarlet spider's poison doesn't do much. Casting "Magic Circle vs. Evil" on your spider should work. You could get her a Belt of Tumbling, but she would have to enter the foe's square _before_ bursting into a swarm if she wanted to benefit from it. Maybe a "feather fall" ring so she can climb on the ceiling & then drop upon a foe before swarm-bursting? Since swarm damage is independent of natural attack damage, I rule that improving the latter won't help the former.
Anything that raises your hp (Con or the Toughness feat) raises your swarm's hp by the same amount, and the same goes for skill points, so Int is more useful for swarm-mongers than for most druids. Area effects are a weakness that can be overcome by the Resistance evolution (see below) and the Concordance faction boons.
Where are you getting "diminutive size" from? All fecund familiars are Tiny.
Improved Familiar is perhaps the most obvious help. It's doubtful you could get away with an entirely different sort of critter, but applying the celestial, fiendish, etc. template should work.
Evolved Familiar provides three useful evolutions: Improved Natural Armor (more than once?), Resistance (probably against fire, which the celestial template doesn't cover), and Magic Attacks. The catch is that you have to raise your Int & Charisma, which might otherwise be dump stats, to 13. Skilled might be worth it for Acrobatics, Stealth, and/or Perception.
A reading of the various combat maneuvers reveals that most are prohibited and/or useless (how often can a swarm of Tiny critters use the push maneuver?). However, I see nothing that precludes the "steal" and "dirty trick" maneuvers. So, if/when your foe gets an AoO against your swarm (despite the spider's high Acrobatics bonus), you may as well try a steal or dirty trick. Better yet, the pilferer archetype gives Improved Steal, so the swarm can steal from multiple targets at once.
My own question is, how much should I invest into my fecund familiar, as opposed to focusing on being a caster druid? If I want to go all in, I will have to sacrifice Strength & other stats to qualify for Evolved Familiar, and that means reducing the swarm's (and my!) hp. And, what feat should the familiar take? Dodge looks good, or am I overlooking something better? Lightning Reflexes or Iron Will?
Human ***** Dwarf
Str 7 versus Str 8
Dex 12 ***** Dex 14
Con 15 ***** Con 15 (+2)
Int 14 ***** Int 14
Wis 15 (+2) ***** Wis 15 (+2)
Cha 13 ***** Cha 8 (-2)
