All good points! Sorry it's still quite unclear at the moment, first alpha version and I'll be working on it :)
If you place a granary stockpile, they'll place all the starting seeds into that so you can see what which crops you'll want to place. For the mushroom farm, you'll have to harvest some mushrooms in the wild first, they're only found in caves inside the mountain but some maps start with caves exposed on the minimap. Fishing isn't in yet so I'll remove the profession for now, but it's coming in the not too distant future to help with your food stocks.
Seems like the food does need balancing a bit, if you plant most of the seeds you start with you should be fine, a good rule of thumb is 10 crop tiles per settler. As more join they bring their own seeds and rations which should last until those new crops could be harvested. It's the only challenge in the game right now :)