It seems like the invocations that use spell slots should be cast at spell slot level without actually burning one of the two slots. This would allow one to create a more castery warlock with a mix of encounter spells and daily spells. It also supports the AEDU format from level one. Bane as an additional daily makes sense and is in no way overpowered. Bane as a daily that also burns an encounter slot is just stupid.