Sending's actually quite bad for this, if it's being cast via scroll.
25 words, from nowhere, unexpected, by someone you barely know, and there's invisible devils after your tribe (the imp, for example)?
And you're not even a magic user?
Their first thought isn't going to be "Gorok's mage friend" and 25 words isn't enough to explain what's going on and ask questions, and expect a response. Doing it with spell slots during downtime is a lot cheaper than using scrolls.
Mina or Sirim casting
scrying (preferably with their own spells, as this might take multiple attempts) on various members of Gorok's tribe, and then trying to cast Message
through the scrying spell (with diplo buffa) seems like it would work better?
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On that note, with the Scrying spell, I'd argue that Mina and Sirim now have "Secondhand (you have heard of the subject)" knowledge of Erinea Doris.
So when Mina and/or Sirim level up, they can try to scry on her with a +5 to Erinea's save, and then cast message through it (with Bestow Insight Diplomacy and other buffs, of course)
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Edit:
@Goldfish
VotM:
Intelligence: You immediately gain a flash of insight,
and gain a number of ranks equal to your
Hit Dice in any skill in which you have no ranks. These ranks last for 1 minute, during which time you can also treat that skill as a class skill.
Bestow Insight:
When casting this spell, choose a single
skill that you have at least one rank in
For 200 Gold to cast VotM, someone could put Bestow Insight [Skill] into a ring.
Which might be handy if we need to
really boost someone's skill. Especially if the spell itself was learnt by Mina/Sirim, as opposed to Shadow Enchantment, since then they (or at least Mina) could put an Extended Bestow Insight into a ring, for times when we need the spell to last for over 10 minutes for a scroll.
Since I don't know what level Extended (via Rod) Shadow Enchantment: Bestow Insight counts as for putting into a RoSS, either 3 or 4.
Anyway, we should by two scrolls of Bestow Insight.
Honestly, for the sake of saving time, rather than getting one scroll of the spells that both Mina and Sirim want, and having them teach each other, it might be worth it to get two copies, so one doesn't need to spend time teacging the other.
At least for cheaper spells.