Is he who is plagued by evil, blasphemous whispers and thoughts—which he neither utters nor believes in—considered an infidel, and will his deeds be invalidated?

Evil thoughts and whispers are forgiven, and Allah does not hold a Muslim accountable for them as long as they are not acted upon or spoken. Hating them is a sign of faith. As long as you have not uttered disbelief intentionally or committed an act of disbelief, you are a believer. Be reassured and ignore these thoughts, seek refuge in Allah, and supplicate to Him to avert them from you.