Learn Hardcore Russian: На конференцию приедут докладчики и участники из разных городов. - Speakers and participants from different cities will come to the conference.

The preposition в is more for being inside a physical space: в комнате (in the room), в магазине (in the shop), в университете (in/at the university as a place).

A конференция is treated as an event you go to (like a concert), so the natural choice is на конференцию, not в конференцию.

What case is конференцию in, and why is that case used?

Конференцию is accusative singular feminine.

Pattern:

nominative: конференция
accusative: конференцию

After на you get:

на конференции (prepositional) = at the conference (location, no movement)
на конференцию (accusative) = to the conference (direction/movement)

In this sentence there is movement towards the conference (people will arrive there), so на takes the accusative: на конференцию.

Could the sentence start with Докладчики и участники instead of На конференцию? Does the word order change the meaning?