Learn Italian: Questo interrogatorio è diventato interminabile.

You could say, Che interminabile che è diventato questo interrogatorio! In this spoken structure, the first che acts like 'How' or 'What a', and the second che is the pleonastic (grammatically unnecessary) one that Italians add for extra emotional emphasis.

Why does diventare use essere (è diventato) instead of avere here?

Verbs that indicate a change of state (like diventare - to become, crescere - to grow, nascere - to be born) always take essere in compound tenses. Because it uses essere, the past participle must agree with the masculine singular subject (interrogatorio), making it diventato.

The word interminabile looks like 'interminable' in English. Is the -abile suffix used the same way as '-able'?

Yes! The suffix -abile (for verbs ending in -are, like terminare) and -ibile (for -ere/-ire verbs) means 'capable of being [verb]ed.' Here, in- (not) + terminare (to end) + -abile (able) perfectly matches the English 'endless.' Remember that adjectives ending in -e use the same form for both masculine and feminine nouns.