1.Stats: Should Klefki get a stat boost, Evolution, Mega Evolution or nothing?
Yes. Klefki has very low BST, and no stats that reach above 100. It can get at most +70 BST, but a smaller boost is sufficient.
Klefki definitely deserves an evolution and a pre-evolution. A Mega should only be considered as a last resort.
2.Should Klefki get more abilities?
Yes. Misty Surge is the most obvious option, but stuff like Magic Guard, Trace, Levitate, Synchronize, Clear Body are all options as well.
3. Should Klefki get more moves?
Yes, moves like Magnet Bomb, Beat Up, Stored Power, Nasty Plot, Cosmic Power should all be available for Klefki. Klefki would also enjoy moves like Simple Beam.
4.What additional typing Klefki can have?
Dark, Psychic and Ghost are obvious options. Electric and Flying are also options.