If there were no cost to mindreading, then it might as well be an extra dialog option. With a significant price, there is much more to the risk-reward dynamic.
I think overall it has little effect with your final level, though you may progress a little slower. People have reported mindreading a fair bit and still ending the game about level 34 or 35 (which is normal).
Shopkeepers do update their inventories, but I don't think anyone has narrowed down exactly when. There are several topics about this already, theorizing it happens every couple character level changes, or after completing significant quests, or clearing certain areas, etc.
I don't know if plants respawn, but after you get the battle tower you can repeatedly send a runner for alchemy plants.
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