Learn Italian: In sintesi, il contributo dell'inventore all'umanità è inestimabile. - In summary, the inventor's contribution to humanity is inestimable.

In sintesi is slightly more formal and literally translates to 'in summary' or 'in brief'. You will often see it in written Italian, presentations, or structured arguments. Insomma is more conversational and versatile, often used to mean 'in short' or even just to express frustration.

Could I use in fin dei conti instead of in sintesi in this sentence?

You could, but it changes the nuance. In sintesi implies you are directly summarizing points you just made. In fin dei conti translates to 'at the end of the day' or 'when all is said and done', which feels more like a final realization or a bottom-line judgment rather than a strict summary of facts.

Why do we use the articulated preposition all' before umanità instead of just the preposition a?

In Italian, abstract nouns and broad concepts like umanità (humanity) almost always require a definite article. Since the phrase is 'to humanity', we must combine the preposition a with the article la (which drops its vowel to become l' before another vowel) to form all'umanità.

Is inestimabile used the exact same way as in English?

Yes, it is a perfect cognate. While it literally means 'unable to be estimated', in everyday Italian it is commonly used to mean 'priceless' or 'invaluable', just like in English. Because it is an adjective ending in -e, it remains exactly the same for both masculine and feminine nouns.