While I can't pretend to have researched it thoroughly I am pretty sure if there were dark secrets about the Dalai Lama they would have been revealed by now as he is by far the most famous and revered guru alive. If you contrast his upbringing with Prem Rawat's you will see that the DL was subject to strict discipline, difficult conditions and an extremely rigorous Tibetan Buddhist education though whether that has any real intellectual value is debatable. And then he dose seem to eb a pretty simple nice person.
Prem Rawat on the other hand had a standard education and was able to remove himself from any discipline by the time has was 13 or so. It's interesting that he claims to have been "giving satsang" for a few years before it was "revealed" to him though it only took him a few weeks to "realise it".
It's not as if the Tibetan elite were always able to keep the Dalai Lamas under control despite the rigorous upbringing as one of them became a playboy and a few were poisoned when they became adults though that was probably because the regents wanted to maintain control and not necessarily because the DLs wouldn't toe the line.