I think Einstein instead of allowing himself to accept something that might have been false in his religious beliefs instead opted for a childlike religion of possibilities?
Einstein, I believe, trusted in a Creator that would appreciate his courage of conviction. :-)
The possibility of mirror-matter is something taken seriously by some physicists, and some of what is printed on the site in your link looks valid; I have to say though, that there's some stuff in your link that looks fishy, though (i.e. the whole deal about the Tunguska explosion - just as easily explainable by an ordinary comet impact). Always remember, the web is an incredibly vast repository of information with zero quality control; always check the sources of info before taking it too seriously.
In any case, mirror matter is an interesting concept - it may well be real (though once again, I'm not an expert on this).