As far as i can tell you need to get the hookshot, to do so you need to complete the main story where you meet the mermaid again, however after you get the hookshot you instantly return to the village and there are some "events" happening there which lead to the end of the game.
Sadly no, you sort of have to grind for it and it doesn't give you many blackberries every day, as far as i could tell you can get up to 5 blackberries per day (mind you i said up to, meaning that you can get less or none at all).
Well the creator of the game (MuseX) has a patreon where they upload news, scenes, ideas and the games themselves as they get released, it also states that he releases games on a monthly basis.
if you need it for what i think you need it (the sinker lure) you can use corn+ for it will work all the same, also as far as i know you can't reverse the process from getting corn+ to regular corn (which is the same except corn+ is better quality)
No we can't use corn+, i have tried that already. It seems that i need corn for sinker lure.
Go near the entry to the mines where Derek is and go past him to the the right of the screen, that path leads to the beach
.... Also note, ninas grandmother never meets me at the well, no matter when i go...
