Q2. What future works have the authors mentioned in the paper "The theory of calculi with explicit substitutions revisited" ?
The authors leave this for future work. Note however that λes-reduction can be translated to the correspondent notion of reduction in this calculus: thus for example App1 can be obtained by App followed by Gc. The authors believe that simultaneous substitutions will be needed to avoid axiom C while some technology like de Bruijn notation will be needed to avoid axiom α ( as in the λσ⇑ -calculus ).