As Enternius mentioned there is not much info about how much Elemental Seal gives , but for me it seems that Elemental Seal adds percentage % of your magic accuracy , I've never seen elemental seal resist unless the mob is immune to particular spell or you're using spell that weak against it e.g: water-based spell against Thunder Elemental.
For your question about Focalization , IMO , Yes it worths the merits.
Much of the choices in group 2 merits depend on your playing style , the way you use SCH , Is Focalization as potent as Elemental Seal ? No , but it's very decent since you can use it every 2 mins , basically you're adding Macc+25 every 2 mins for your next black magic enfeebling, elemental, or Dark Magic skill .
Personally I went 3/5 Focalization , while some people says 5/5 or bust , for me I merited the way I play the job & I have enough Macc/skill gear to make it up for the 10 macc .
If you don't wanna merit Focalization you still have great ways to get around it e.g :
1) Alacrity + Voidstorm + Klimaform while equipping Argute Loafers , then cast the weather of your choice according to the spell you're using , that way you can get Klimaform recast almost as low as 1 min (I'm not sure exactly how much u can cut off the recast but it's really very low) , so you would almost have permanent Klimaform , considering you only use Alacrity .
2) Maxing Stormsurge , while under the effect of Hailstorm you would have INT+7 (which gives magic accuracy anywhere between 3.5~7 depends on your dINT "difference of INT between you & of the mob") , while it's not *** effective as previous way , it's great over-time.
3) Parsimony is your friend , & gear up toward skill "like if you're casting on something really resistant e.g Tiamat" , in best cases if unresisted you will cast as much twice as your friendly neighbor BLM with less damage but almost stable damage , in worst cases if resisted you would have spent less MP / resisted damage lets assume half resist , fair trade .