Did the Kongolese actually keep written records in Swahili rather than Kikongo? Etc.

Nope. Swahili is a lingua franca used in portions of the DRC today, but Swahili and Arab traders did not reach the eastern parts of the congo basin until the mid-19th century. The use of Swahili for the Kingdom of Kongo is very anachronistic.

Rather, the Kongo kingdom was highly influenced by their contact with Portuguese explorers from the mid-1400s onwards. The influence was substantial enough that the kings converted to Catholicism and adopted portuguese names.

As a corollary of that influence, much of our knowledge about the Kongolese army comes through the accounts of Portuguese administrators or missionaries. As you would expect, these writers use Portuguese terms rather than Kikongo ones.

So, in academic literature the Kongolese shield bearing heavy infantry is usually called Adargueiros while the conscripted archers were referred to as Mobatas.

John Thornton proposes that the village chiefs constituted the lowest level of Kongolese nobility, reflected in the title fidalgo mobata, and that these men would have constituted a core of consistently trained professional soldiers that would become the Adarguieros.^1

Edit- I don't know of any accounts that claim to give the Kikongo term for these elite troops. From a gameplay point of view, it is understandable that developers would not want to give Portuguese names to the Kongolese elite troop unit and instead substitute something more "authentic".

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1 Warfare in Atlantic Africa, 1500-1800 by John Thornton, pp 114