Would pyrrhotite actually decompose to magnetite preferentially over haematite?

While this paper is based on pyrite, the same processes apply and suggest that the products are haematite (FeO / Fe2O3), not magentite (Fe3O4) https://www.911Metallurgist.com/C/Pyrrhotite%20deposition%20through%20thermal%20projection%20to%20simulate%20iron%20sulphide%20slagging%20in%20oxyfuel%20combustion.pdf

Heating Pyrrhotite at 320 degrees C. will destroy its magnetism.

Heating in the presence of Oxygen (no water) will have the following reaction equation (balanced) ; 3FeS + 5O2 = Fe3O4 + 3SO2

This is the process that used to be used at INCO in Sudbury, Ontario, Canada to convert FeS to Magnetite, which was then pelletized and sold as Iron Ore.

So is it possible to directly feed FeS for pelletization. The SO2 we can remove through FGD.

FeS must be roasted in presence of Oxygen, it is an exothermic reaction (it burns).

In 2015 no plant can just decide to burn a sulphur compound then vent the SO2 to atmosphere. Given the need for a pure oxygen feed the proposed burning could produce a quality gas stream for a sulphuric acid plant. You have not mentioned any daily rates but even the cheapest acid plant would be in the tens of millions IMHO. It would also require a reasonably close market due to shipping costs.

There are wet hydromet methods that can produce elemental sulphur. What are your overall economic parameters?

We can absorb the vent SO2 to produce gypsum as well. The nearby market are yet to be reviewed.

When I took my apprenticeship in a major corporation we were taught to collect all of the economic parameters relevant to the project. As vendors of both metals and chemicals the company had a lot of economic data. What was not available for a proposed project would be the subject of questions sent where relevant in order to complete an assessment?

You now suggest that you are asking others about the chemical results of burning FeS, in itself a well-documented subject, but you have also not researched your local cash flow options related to disposal of the sulphur side of the reaction. That is not engineering; at best I'd call it a second year student exercise. How many years of education and practice do you have? Is your question just a personal request or for a company quest.

Without getting into process economics & per your original query:

Pyrrhotite exists under multiple polytype crystal symmetries of the type Fe(1-x)S; reactions of one type:

3Fe7S8 +38O2 = 7Fe3O4 + 24SO2 (>300C)

Fe3O4 = Fe2O3 +FeO (>1500C)

Fe (1-x) S as fine powder could react with SO2 gas over 150 centigrade in aerated environment at 0.6 atm. Try on dryer plate