AN USURPER is an illegitimate or controversial claimant to power, often but not always in a monarchy. This may include a person who succeeds in establishing himself as a monarch without inheriting the throne or any other person exercising authority unconstitutionally.
Usurper is a very common word and very appropriate for the leader of a coup d'etat. It can also in the plural apply to a group or a movement, but seeing as all groups tend to have a leader, Usurper makes sense in the singular as the focus of the usurpation.
The term is very established and old and is a bit antiquated for modern precise use, it's pronunciation is not the same as Georgian english, as it has vague, romantic and literary drama connotations perhaps. The Times for example may chose another term because Usurper has existed since the middle ages, and was more frequently used previous to WW2, because it still has a royalist and imperial tone of times past. It is more appropriately used for coup leaders of previous centuries, where it fits perfectly. The most current term is Coup Leader, Leader of N faction, Ex General, Warlord, Insurrectionist, Recent Accessor to power, New leader, Coup Plotter, Revolutionary, which are mostly 2-3 syllable terms and highly practical.
Usurpation: The wrongful seizure of something by force, especially of sovereignty or other authority. Trespass onto another's property without permission. A taking or use without right.