Welcome to the Shard! And thank you for reading my magic system. I'll try to answer your questions by order.
1. Harvest doesn't exactly leak, but Fertilization does take some Harvest, and it's a passive use of the magic system. So it sort of decays with time, I guess.
2. Society and politics are the most underdeveloped parts in this world, unfortunately. I guess there might be some tension between different sapient species, and between different mindgardeners. The thing is, the mindgardeners are generally animals, though there are exceptions. They don't live within human - or any other sapient species - society, or even much with each other. They don't really act that much like animals, admittedly, but they don't act much like sapients, either. They're not much liked though. And Wanderers mostly live in exile, unwanted by the regular society. How severe are the tensions? Different species of the Faunologos have different Centers, which function somewhat like Intent in the Cosmere, I guess. Due to that, creatures with opposing Centers are not going to like each other. I can't really say how severely, but they might fight each other whenever they meet. Which, admittedly, isn't too often.
3. No. The mindgardening abilities are something natural to the Faunologos, not inherent for sapient creatures. It's due to the symbolic nature of the magic system - sapient creatures don't symbolise anything in particular, they can represent everything, in theory, since they contain all the potential Centers naturally. That's why it doesn't come naturally to sapients - about animals In general I still need to think up why the Faunologos are different from regular animals.
4. No fabrial-esque, I'm afraid. Mostly, I intend to have them with more medieval technology, like a traditional fantasy world. I probably should consider changing that, though.
5. For now, not much notable figures. Though I really need to think about it more.
Thank you for asking those questions! When I'll have more time to consider, I'll think of better answers. As I see it, people asking questions is what makes me see the holes and problems with this magic system, and helps me clarifying things. So, no matter how poorly-thought you think those questions, they are actually quite helpful.