You do NOT need iron/steel to have a successful fort. As others have mentioned, iron is largely a sedimentary stone. One of its three ores can be found in volcanic stone, but again, only near the surface. You will NEVER find deep iron. When you smelt tetra, you get a little silver. That silver makes great warhammers that are aproximately as lethal as steel edged weapons. The copper armor is defnitely low grade, though. Its still better than wood/leather/bone/shell stuff. Use wood for shields though.
Never enable both fishing and fish cleaning on the same dwarf. They tend to just leave the fish on the ground to rot instead of taking them to be cleaned. Those "lakes" are just murky pools, though. Actual lakes are giant and never fit within a standard embark square. Murky pools are stagnant swamp water, and if they even have fish will quickly run out.
Dwarves eat exactly twice a season. So one dwarf will only need t use the dining room 8 times a year. However, if you have tables set up elsewhere (like for a library or hospital), they may eat there too. Offices (like for the manager) will also be eaten at by the owner of the office.
Its hard to beat a good plant gatherer, actually. In areas of dense vegetation I've seen a level 5 one bring in 100+ units of plants within just a few weeks. Then in hte summer they can strip hundreds more food off of fruit trees with a stepladder. A more passive way is to just leave cage traps throughout the wilds (watch where animals usually move and place traps there) or on your open door to the caverns. 20 traps in a narrow hallways leading back into your fort can drown you in cavern meat (and bones/leather). Eggs and non-grazers I found exploity, as they require no actual food. Pigs locked in a stone room for generations still create milk and bacon out of... rock dust? However the best egg-layers are reptiles hands-down. Its very easy to capture a breeding pair of cave crocs from the caverns, and they'll lay clutches several times larger than a turkey. Same with above-ground crocodiles.
I wouldn't worry about a militia before the first caravan, but by your third migrant wave you should have more than enough dwarves to set aside some for training. If no one has any skills, they will all train very slowly. However, once you get an established core of soldiers, you can start breaking that first squad apart to seed o thers. A single experienced warrior at the head o a new squad can speed up training with his high levels of "organizer" and "teacher" for drills. Don't give them ull metal armor either, without training it greatly encumbers them. Start with leather armor and a metal helm+wooden shield. I think they need no less than 3 levels of "armor user" before they can handle a full metal set of equipment.
Again, iron is neither guaranteed nor needed. When you embarked on a site it will have told you if a rough sketch of hte metal profile. "Shallow metal(s)/Deep metal(s)" If it just had a single shallow metal, then that tetra is all you get. Iron will never be deep. You could very well be in a sedimentary layer too (are the layers of stone made of things like chert, rock salt, siltstone, mudstone, shale, chalk, etc) The sedimentary layer is also the default one unless you spawned in an area with high volcanism (near a volcano or magma vent). If that's the case you'll see layers of basalt, rhyolite, obsidian, dacite, andesite.
You can move over one more level and individually select seeds with enter. I'd actually start by turning off EVERYTHING in the stockpile, then flip to the bottom of the seeds list (the underground seeds are near the bottom) and individually turning them back on with just "enter" over the names.
Milling and processing to barrels/bags will not destroy seeds. The products cannot be eaten raw, but they do preserve seeds.
However, I'd like o mention here that cooking IS NOT REQUIRED. Your dwarves will just as HAPPILY eat raw pig guts on top of a stone table as they would a masterfully cooked prepared meal of eggs, flour, dwarven syrup, and strawberries. Cooking is mostly to consolidate stockpile space (all prepared foods stack together where the ingredients might not), inflate value (its just stupid), and make inedible foods edible (most seeds, flours, quarry leaves, oils, milk, honey, syrup, sugar, etc). Other than seeds, you have to go out of your way to generate the processed foods that need cooking. Most foods will be happily eaten raw. More importantly, you have better things to do with your dwarf-labor than setting aside a cook, a ton of bags, extra barrels, millers/plant processers, and the extra hauling all of this needs. Those are people that could have gone into your first military squad. I generally won't make a cook until after the first year or later.
Not even remotely. A 1-tile wall stops nothing. Its not even a matter of height, unless you want to build a 20+ z-level wall that enemies become exhausted and fall off of before cresting the top. Material is more important. Rough stone and dirt are easy to climb, blocks and bars are still manageable climbs, but smoothed natural stone is impossible to gain a foothold with. Build that simple wall, but then dig a 3-wide moat down into the first two natural stone layers and smooth those. Or build 3-high wall and build a "ledge" two tiles wide around the top of it. Enemies can't reach that far up behind them and pull themselves up over the top.
However, you never need to actually build defensive walls. Even if you want to graze animals and make surface farms. Animals will happily graze on cavern moss underground- just open up the caverns and immediately wall the entrance back off (a single tile breach is all you need). Now, cave moss will grow on undergorund dirt. Sheep will eat this with no problems. For farms (or also pastures) channel into the dirt, then roof over the pit. Now dig underground tunnels leading to it. You now have SECURE aboveground farms/pastures... they don't need light to grow. Surface installations are just for your fun. Even archery towers or marksdorfs shooting off of walls will loose range from the extra heigh, and siege engines are completely incapable of firing up/down z-levels.