1 If a character is a target of an attack that causes knockback but another figure and a wall create a diagonal wall behind said character...does it still get knocked back through the diagonal or does it sit still?

Example: (Robin (R), Catwoman (C), Hawkgirl (H), Blocking Terrain (B))
xxxH
CRxx
xBxx
Hawkgirl causes knockback to Robin...Catwoman and a wall behind him...does he get knocked back through the wall?

2 If a character shows L/C on their dial when getting knocked off a roof, do they avoid taking the 2 damage from falling?

Yes. That diagonal, between Catwoman and the blocking terrain, only blocks LOF. It does not impeed movement; Robin could walk through that space (or get knocked through it) just fine.

The PAC description of Leap/Climb and the Icons rule book discussion on knockback say nothing about Leap/Climb preventing knock back damage, so no.

PAC LEAP / CLIMB (optional): When you give this character a move action, it automatically breaks away and ignores the effects of characters, hindering terrain, elevated terrain, and outdoor blocking terrain on movement. When you give this character a close combat action, this character may target a grounded or soaring figure regardless of the target’s elevation or flight mode.

Rule book (page 21) Knockback off elevated terrain. If a non-flying character is knocked off elevated terrain, the knockback path stops in the first square beyond the boundary of the elevated terrain and the character is dealt 2 knockback damage. If a character already occupies the first square beyond the boundary of the elevated terrain, the knockback path stops in the first unoccupied square (leaving the character on elevated terrain) and the character is dealt 1 knockback damage.