Can I use Sceptile's "Nurture and Heal" ability to attach a {G} Energy to a Pokemon that doesn't have any damage on it?
Yes, you can.
Source: Primal Clash FAQ (2015-02-05), TPCi Rules Team (2015-02-05)
If I use Sceptile's "Nurture and Heal" ability, is that an extra Energy card attachment or does it count as my one attachment for the turn?
It is an extra Energy attachment. You still get to attach one other Energy card as your normal attachment for the turn.
Source: Primal Clash FAQ (2015-02-05), TPCi Rules Team (2015-02-05)