Learn Spanish (Latin America): ¡El cono se iba a desmoronar en mis manos! - The cone was going to crumble in my hands!

Not necessarily. The structure iba a + infinitive (the imperfect of ir + a + infinitive) is used here to express an action that was on the verge of happening in the past. It usually implies that the event was interrupted or avoided, meaning the cone was about to crumble, but the speaker likely stopped it from happening by eating it.

Why is the pronoun se used here?

The se belongs to the verb desmoronar. The pronominal form desmoronarse is used when something collapses, decays, or falls apart on its own. If you left out the se, it would sound like the cone was intentionally going to crumble something else!

Can I attach the se to the end of the infinitive instead?

Yes, you can say ¡El cono iba a desmoronarse en mis manos!. With verb phrases like ir + a + infinitive, you always have the choice to place the pronoun either before the conjugated verb (se iba) or attached directly to the end of the infinitive (desmoronarse). Both sound perfectly natural.

Is desmoronarse only used for physical objects like ice cream cones or buildings?

No, it is a very versatile word! While it describes physical objects crumbling, it is also incredibly common to use desmoronarse for abstract concepts. You can use it when a person has an emotional breakdown, or when a plan, marriage, or system completely falls apart.