Learn French: Quiconque trouve l'intrus doit le signaler avant qu'il soit trop tard. - Whoever finds the intruder must report him before it is too late.

It actually acts as the subject for both trouve (finds) and doit (must). Quiconque functions as a single pronoun meaning 'anyone who,' so you don't need to add an extra pronoun before the second verb. It always takes a third-person singular verb.

Why is the pronoun le placed right before signaler rather than before doit?

In French, object pronouns go immediately before the verb they are logically attached to. Here, you are reporting the intruder (le signaler), not 'musting' the intruder. When you have a conjugated verb followed by an infinitive (like doit signaler), the pronoun almost always goes right before the infinitive.

Why is the verb at the end soit (subjunctive) instead of est?

The conjunction avant que (before) always requires the subjunctive mood. This is because 'before' an action happens, that action is still a hypothetical future event, not yet a confirmed reality.

Why do we use il soit instead of ce soit for 'it is'?

When talking about time being early or late, French uses the impersonal pronoun il, just like in the fixed phrase il est tard (it is late). You cannot use ce (as in c'est) for these specific expressions of time.