If your sources spelled her title Comtessa, then what's written below is a moot point. It's best to stick with historical records, even if there are incorrect spellings.
In French, Countess is spelled Comtesse just the same as Duchess changing to Duchesse. The "a" ending may have been used by sources more familiar with Italian: Contessa (without the u).
The thought occurs that the wisdom of Geni adding a field for the language/s the person spoke or is likely to have spoken should be discussed/considered. For example I am an American English first language speaker writer. One set of grandparents are likely to have been truly bilingual English/German. While 'their' parents etc were likely German only speakers though. The other set was English only but 'their' grandparents might well have been Welsh first language with english second. Lotta' guess work of course. Cheers