What is the ruling on ignoring the whispers of urinary incontinence (waswas al-bawl) and not inspecting one's garment for yellow stains? And must past prayers be redone if such stains were present?

You must ignore intrusive thoughts and not care about them, as there is no better cure for them than that. The yellow color may be sweat or something else. Do not judge that something has exited unless you have certain conviction, such that you can swear by it. This is the saying of Sheikh Ibn Baz and others. If you are certain that something has exited, you must perform istinja' (ritual cleansing). Otherwise, the default is that you are absolved. You are not required to investigate; rather, assume the default that nothing has exited. If you are certain that something has exited but doubt whether it is madhi (pre-seminal fluid), maniy (seminal fluid), or something else, then choose and assign it one ruling.