Mr Aye Dee's Nuggets On the Abduction/Release Of Dapchi School Girls. #DapchiGirls:

In the last few years, a split happened in Shekau’s ISWA, this was because the more ‘civilized’ members thought his scorched earth policy of indiscriminately killing everyone by any means necessary, including the use of child bombers was barbaric

There was a split and Shekau lost control of the entire organization, he still has men under his command , but another faction under Al-Barnawi (II) emerged. Al-Barnawi's group keeps waging jihad but in a more 'civilized' way (in their opinion, not mine)

The abduction of the Dapchi Girls was not in line with this pivot to 'civilized' insurgency, seeing as most of the girls were already Muslims, not Kafir (nonbelievers) and thus, from the point of view of the abductors, not eligible to become captives and slaves

The logistics nightmare of keeping 100 pre-teen and teenage girls in what's at best military camps, girls that they could not touch sexually based on their interpretation of Islamic injunction, girls who also happened to be Muslims became an issue.

Consequently, the goal was to get rid of them as soon as humanly possible, and when FG reached out, they agreed to return the girls and did just that. 

Consequently, insurgents driving into Dapchi in trucks, it's obvious they were guaranteed safe passage as has been the case during every other exchange or 'contact'

If FG had attacked the convoy on its way to town, the girls would have been killed, if FG attacked the convoy as it was leaving town, trust would be broken and we would go back to the main issue between BH and GEJ's admin, lack of credibility on FG's part.

It might be hard to believe, but the insurgents, even those led by Shekau tend to always keep their word. 

Anyway, those who want to engage in conspiracy theories can continue doing just that, but hopefully, as someone who is somewhat familiar with the subject matter, I've been able to shed some light.

Source: @MrAyeDee