Why do you think students were asked to complete the elements, mixtures and compound sheet at the start of the lesson?

To find out what they already know and identify any misconceptions students might have. A good understanding of elements, compounds and mixtures are important foundations for learning about chemical reactions.

What's the advantage of asking students to draw diagrams showing the particles of iron and sulfur at the start of the practical activity rather than doing it towards the end of the lesson during the final plenary? 

It helps them to make connection between their macroscale observations and what is happening at a sub-microscale when they make observation and draw models at the same time.

Would you expect the students to get the diagrams of the arragement of the particles in iron and sulfur correct?

As always it depends on both the ability and experience of the class.

However, it is not uncommon for students to draw both set of diagrams exactly the same; thus implying that particles of iron and sulfur are exactly the same, despite the different observations. This then leads to difficulties when they come to draw a particle diagram of the mixture.

If students are provided with powdered sulfur some of them may draw a diagram representing particles in the liquid state indicating an insecure understanding of the differences between solids and liquids.

After the demonstration, why do you think that the students were asked to devise a role play that would involve the whole class acting out the reaction rather than just drawing more particle diagrams?

Role plays are useful to get over the idea that during a chemical reaction the actual particles of iron or sulfur atom do not change. They rearrange and join (or combine) to make particles of a new substance, in this case, it is iron sulfide. If the boys take on the role of particles of iron and the girls the role of particles of sulfur, at the end of the reaction, the boys and girls are still the same, it's just now they are joined to each other.