Can I use Forest Seal Stone's "Star Alchemy" Ability if Garbodor's "Garbotoxin" Ability is activated?
Yes, you can. Star Alchemy is not an Ability the Pokemon has, but it's an Ability the Pokemon has access to.
Source: TPCi Rules Team (2024-04-18)
If I'm using Scizor's "Punishing Scissors" attack and one of my opponent's V Pokemon has Forest Seal Stone attached, does my attack do an extra 50 damage for it?
No, the V Pokemon does not *have* the Star Alchemy Ability, it only can use it.
Source: TPCi Rules Team (2023-09-28)
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)