Joseph,
1. His conversion would only be invalid if he was an apikoros at the time of his conversion.
2. Did Rav Eliashiv change his mind after the Langer case? he resigned form the Rabbanut Supreme Rabbinical Court after Rav Goren annulled the conversion of a man who continued to go to church after his conversion and thus cleared his wife’s children from her second marriage (after #1 disappeared) from mamzerut? On what level does one have to believe that the age of the world is 5,777 years? Does the count start with the Big Bang, people starting to speak (that is to say, when they became peopel), when they started to write? Relative to what (see “Age of the Universe” by Dr. Gerald Schroeder on the Aish HaTorah website).