First of all, Hazrat Mirza sahib (as) didn’t bring a new scripture nor did he make such claim.
Many of signs of his truthfulness are listed in Haqeeqat-ul-Wahi. Available online. you can read and see for yourself.
What miracles or signs did mr. rashad khalifa present in support for his truthfulness ? If I am not wrong, he rejected two verses of quran on numerology basis. Hazrat mirza sahib (as) said that Quran is in its pristine form and no verse can be rejected.
How long did he live after claiming to be a messenger of Allah?