A trial of ants is marching across... Is gramatically correct, right?

Well, it depends what you mean by subject, right. Syntactically, the noun phrase, the trail of ants, is the subject of the sentence. Subject verb concord would be based on the head noun phrase the trail. How you handle the verb, singular versus plural, does have something to do with how you interpret the whole NP, the trail of ants. I would treat a flock of sheep as plural, but a herd of bison as singular: e.g., "A flock of sheep were cropping my lawn", but "a herd of bison was blocking the road".

Ceci n'est pas un seing.