regarding the validity of 'mahayana' sutras, i see historical facts vs insights. facts are that which follow a solid and 2 dimensional linear timeline. insights are that which slips through time in the 3rd or 4th dimension(String Theory or Ten Dimensions?).
enlightened beings would have transcended the time barrier. hence sutras do not have a time indication. they had always been "Thus I have heard". they can be anytime. it's commonly known that Shakyamuni passed on about 2500 years ago, while from the chinese calender, it's about 3000 years. from these we already see that time is dependend on people, let alone to people's mind.
facts are for undeveloped conventional mind, while insights are for the developing/developed mind.
ps: for an interesting info, regarding String Theory or Ten Dimensions
http://www.tenthdimension.com/medialinks.php/\