Learn Finnish: Palkkasimme mitä oma-aloitteisimman työntekijän. - We hired an exceedingly proactive employee.

The lesson mentions using the nominative case for this construction, but here we see oma-aloitteisimman työntekijän. Why are they in the genitive-accusative case?

While the dictionary or basic form of the absolute superlative is mitä + nominative adjective + nominative noun, the adjective and noun must actually decline to fit their grammatical role in the sentence. Since the employee is the total object of the completed action palkkasimme (we hired), both the adjective and the noun take the genitive-accusative ending -n. The word mitä always stays exactly the same.

How is the long adjective oma-aloitteisimman formed?

The base adjective is oma-aloitteinen (proactive or self-starting), which is a compound of oma (own) and aloite (initiative) plus the adjective suffix -inen. Words ending in -inen change to -is- before the superlative marker. Adding the genitive-accusative ending -n triggers consonant gradation, softening the superlative marker from -impa- to -imma-, resulting in oma-aloitteisimman.

Could we use the partitive case (mitä oma-aloitteisinta työntekijää) instead for the object?

Yes, but it would change the meaning. Using the total object työntekijän means the hiring process is complete and successful. If you used the partitive työntekijää, it would imply an ongoing or incomplete action, translating more like 'we were in the process of hiring an exceedingly proactive employee'.