Keep in mind that Vol’jin didn’t meet with Rastakhan, but with the Prophet Zul who was passing himself off as the representative of Rastakhan.
You need to understand the kind of balls it took for Vol’jin to choose to stay loyal to the Horde in the face of what Zul was passing off as an edict from the King.
The Zandalari are considered the cultural and spiritual leaders of all trolls, keepers of sacred knowledge and the priests and scholars of the most ancient traditions. To put it in a human perspective, Rastakhan was the ‘pope’ of being a troll and Zul was his ‘archbishop’ and he showed up declaring a holy war to retake the promised land from the ‘heathens’ who were now occupying it.
As to the topic itself, yes it’s spelled out several times in game that Zul isn’t actually acting on orders from Rastakhan, and had actually manipulated the King into giving him what support he had simpler because Zul was the most senior troll on the Zan’chuli council and a proven prophet of great power and insight. Zul lied about Zandalar sinking into the sea to get the fleet of ships he first used to rally the lesser tribes to his cause and then later used to invade Pandaria. Zul’dazar plans, however, reached far beyond simple conquest of Pandaria and instead revolves around a plot to learn the Mogu’s secret of reviving ancient kings so he could resurrect the first king of the Zandalari as his puppet so Zul could assume control of the throne directly after murdering Rastakhan.
Now from the Alliance perspective obviously Zul was dangerous and imprisoning home was probably the best thing they could do. From the Horde perspective Zul was not to be trusted, but he might be used as a bargaining chip and as a sympathetic voice on the Zan’chuli Council were the Horde to rescue him.
It probably was wrong of the Alliance to arrest the Princess of the Zandalari, if only because she had committed no crimes against the Horde or Alliance at the time that we know of. Taking her into custody is one thing, but locking her in the stockades seems like a really time dead move by Anduin, and almost intentional - as though he was expecting someone to try and come rescue her.
It’s possible Anduin was playing a bit of a political game and took Talanji knowing Sylvanas would try to rescue her and had heightened the security in Stormwind to capture her would be saviors when they showed up. However, we’ll never know if this was the case, as if it was then it was a dramatic failure and it’s unlikely Anduin would freely admit to gambling with so many Alliance citizens lives.