Wandermare's ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.

If an effect refers to a card, spell, or permanent that has an Adventure, it won't find an instant or sorcery spell on the stack that's been cast as an Adventure.

An effect may refer to a card, spell, or permanent that "has an Adventure." This refers to a card, spell, or permanent that has an adventurer card's set of alternative characteristics, even if they're not being used and even if that card was never cast as an Adventure.