In preparation of the Void quest...

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The rules regarding doing quests as a group can be found here:

If two people are participating in a quest, the word requirements should increase by 33% (so if the quest requires 10,000-20,000 words, it requires a total ~13,333-26,666 words between two people). For each extra person after that, add 66% of the word count requirement (so three people requires 20,000-40,000 words total, four people requires ~26,666-53,333 total, etc). These requirements may then be fulfilled as a group, and each member will receive the rewards individually. 

So, in your case, the word count for the entire group would be 53,330-106,660, depending on the writing quality.

As for the storyline idea, one thing that immediately strikes me as a potential problem is that your plan seems to handwave the actual journey to the mountains. Crossing the Fields is an arduous and treacherous trek, where characters are constantly threatened by the elements, the wildlife and the deceptive and uneven terrain. It'll likely take your characters weeks, perhaps even months to reach the mountains in the first place. If you wish to complete this quest without issues, you shouldn't neglect this part of the quest.

Furthermore, when writing the group's encounter(s) with the blue dragonflight, keep in mind that their attitude towards travelers varies significantly between them. Sapphiron in particular is quite reclusive, so he is unlikely to be welcoming to visitors. This isn't to say that he or another dragonflight member would be outright hostile or that they wouldn't be impressed by you being able to overcome the enchanted caverns. But the party will have to overcome a rather chilly reception in order to arrive at their goal.

However, as long as you keep those two factors in mind, I don't see an issue with your proposed plan. Consider it approved.