Can I use the VSTAR Powers of Earthen Seal Stone or Forest Seal Stone if they are attached to Pokemon-V that have not evolved into a Pokemon VSTAR yet?
Yes, that is correct. But keep in mind that you still can't use more than one VSTAR Power in a game, even if you evolve the Pokemon V into a Pokemon VSTAR later.
Source: Silver Tempest FAQ; TPCi Rules Team (2022-11-10)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH12_154
SWSH12_154
SWSH12_156
SWSH12_156