1.Stats: Should Seviper get a stat boost, Evolution, Mega Evolution or nothing?
+82 BST can be given.
An evolution is necessary.
A Mega would be a waste.
2.Should Seviper get more abilities?
YES. Strong Jaw or Hyper Cutter would be good, so would Intimidate.
3.Should Seviper get more moves?
Recover and Leech Life would be nice.
4.What additional typing Seviper can have?
Dark, Ground, Fire are options.