No matter which option you choose (with Gascard's help or without), there is no way to save your mother from her unfortunate fate. Before Leandra dies, she tells Hawke that she is proud of them. ... Hawke can choose whether to reveal the manner of Leandra's death or not.

Is gascard DuPuis the killer?

Gascard DuPuis is dead

As he couldn't have been responsible for the shade attack on Emeric, the conclusion is that he was not the killer; the only storyline impact is the restriction imposed in the later quest, All That Remains.

Is there a way to save Bethany in Dragon Age 2?

No, you cannot keep Bethany in your party. She will always leave. However, you can determine her fate. If you are a warrior or rogue, do not take her with you at the end of Act 1 unless you also bring Anders.