Can I use Grand Tree to search my deck if I don't have any Pokemon in play that can evolve further?
No, you may not. There must be a known valid evolution path in order to use Grand Tree.
Source: Stellar Crown FAQ; TPCi Rules Team (2024-09-12)
Can I use Grand Tree to find a Stage 1 Pokemon that has no known Stage 2 (i.e. using Grand Tree to evolve Scyther into Scizor)?
Yes, you can search for a Stage 1 Pokemon that has no subsequent Stage 2.
Source: Stellar Crown FAQ; TPCi Rules Team (2024-09-12)