Is it possible that Fredo thought that the murder of his brother was a possible consequence of his involvement with Roth, but was willing to pay that price for what he perceived as his own advancement?

This would make him as evil as he was stupid.

Moreover, I always found it curious that he knew that the senator pursuing Michael belonged to Roth, but Michael (so late in the game) had no idea and appeared thunderstruck upon hearing the news.

How would Fredo get acquire this information and why would he be entrusted with it?