Why was the witch so tempted to cross the mirror and get Veer albeit it was getting Jai's soul in exchange?

In 1920: London, Jai came to know that the only way he could save Veer was to exchange his soul with Veer's to that witch.

On the day, when he prepared to confront that demonic witch, Jai asked Shivangi to hold tight a coconut in front of the mirror which acted as a portal to the other dimension so that the bodily from of the witch might not enter the mundane world to get hold of the dying body of Veer.

He also gave many other instructions in order to save Veer from the witch.

He finally stabbed the witch with that Celtic dagger, which was meant to kill it. But in spite of this, the witch without seeming to feel any hindrances did go ahead to cross the mirror; when she was about to do it, Jai broke the mirror.

The whole point was the sacrifice of Jai's soul for the sake of Shivangi and his spouse.

I couldn't comprehend some of the elements of these events.

If the whole point was to sacrifice Jai's life, why did he tell Shivangi about the coconut and all that stuffs? I mean those stuffs were meant to stop the entry of the bodily form of the witch. But why would the witch even do that if it was getting a soul in the form of Jai's? The witch, as Jai's teacher asserted, would sleep only after taking a soul; it was getting Jai's; so why would it go after Veer then? The whole point of crossing the mirror and coconut seems to be irrelevant to the development of the events.

Also, what was the main purpose of the third Celtic dagger that Jai was holding in the other dimension? It was meant for stabbing the witch to end its existence. But as we saw, it was clear that the dagger didn't affect the witch at all; it was still crawling ahead to cross the dimensions.

Could anyone shed some light on these points?