Does the intention need to be renewed for each prayer when combining them, and is the prayer valid if the intention was not made immediately before the opening Takbir?

The questioner is afflicted with waswasa (whispers/obsessive thoughts) regarding intention (niyyah). His past prayers, about which he doubted, are valid, and he is not required to repeat them. As for future prayers, he should be keen to have the intention present within them. Intention is simply a person's aim towards something and their knowledge of it, as Sheikh al-Islam Ibn Taymiyyah said: "For intention follows knowledge; whoever knows what he intends to do has the intention for it without choice." Every action performed by a rational person must have an intention. So, if he knows that he will pray Dhuhr and Asr together (combined) and resolves to do so, then this is the intention, and he should not pay attention to waswasa after completing it.