yeah i luckily was preaty fine since i constantly save in games that have the slightest chance of loosing progress...i mean like 3 times with 0 progress
i have one question tbh, where can i find the fifth sunken coin? i went thru the whole map like 3 times already i have over 30 hours in this savefile and yet i cant find the last coin (i have the one in the mermaid area the one next too the mermaid tower the one by the farm and...uhhhh...one more forgor from where tho)
Congrats, hope you enjoyed your time with the game^^
The Mystery Key is for a ultra hidden area. Finding that secret is pretty much impossible. Or else it wouldn't be called a Mystery Key after all. In that area is nothing that count towards the 100%, it's just a bonus, so not much to worry about.
As for the bag of coins, I would recommend walking around in town, there might be a new cutscence somewhere.
The farming system is pretty simple to provide a bit of bonus items over time. Once a seed is planted it, you need to be in Alraune form to make the soil healthy for it to grow. Afterwards it grows over time and can be harvested every once in a while.