Should I Help Roderick Witcher 3?

Cut them all down and talk to Roderick to learn something isn't right in regards to the hostage and Syanna. You can help him or not afterwards, then carry on, taking out guards as you go. There Can Be Only One info: helping Roderick here counts as part of the chivalric value "compassion".

Did Roderick know about the blackmail?

Roderick had no knowledge of the blackmail.

How do you get the good ending in Witcher 3 Blood and Wine?

There's no way to keep everyone alive in Blood and Wine, but if you want the expansion's 'best' ending you must kill Detlaff and save the sisters. To achieve this, seek out Syanna during the Night of the Long Fangs, enter the Fairy Tale world, take the ribbon, and defeat Detlaff.