May I suggest you read "Guns, Germs and Steel" by Jared Diamond, which looks at the history of why certain areas of the world developed before other areas, including in particular the development of agriculture. Might help you to realise why certain things succeed and other things fail.

In terms of the grasses, hunter-gatherers went out picking seeds off grasses. But only those grasses that had palatable seeds, and which had a large enough yield to be worthwhile. They preferentially picked the larger seeds. They preferentially picked seeds that didn't fall off as soon as they were ripe (as was the generally the biological tendency of the species), because otherwise they would not have been there to pick them. They dropped a few on the way home, so that quite by accident they found themselves growing them near home, and accidentally selecting them for desirable properties. Why did those Middle Eastern crops come to dominate - because those are the crops that automatically selected themselves for being suitable. The Americas only had one grass species with those suitable properties, which is now Maize, whereas the Middle East had 4 in a small area. Yes, the Canadians have "wild rice", but it doesn't lend itself to agriculture to the same degree. I think there is one British native species of grass with a reasonable yield of seed, I did know what it is but I've forgotten what, perhaps carex elata. Small seed size isn't necessarily a problem, the Ethiopian/Somali staple is teff, and that has tiny seeds.

Edibility is going to be crucial. Nile Perch were introduced to Lake Victoria, which was a disaster because the local people didn't like the taste. Reforming the agricultural sector in Madagascar is a huge headache because the population, substantially of SE Asian extraction, think that rice is the only thing to eat, yet much of the country is much more suited to to maize, wheat, etc. You need your alder seeds to be so edible that they fetch a premium price. The yield of a tree is clearly going to be less than the yield of a grass, because a tree puts a lot of energy and biomass into the wood. So unless you get a premium price, you aren't going to earn enough money per acre. Especially with the higher cost of harvesting from a tree. What do olives and hazelnuts and the like cost in comparison to cereals? Olive oil costs a lot more to produce than corn oil and sunflower oil and rape-seed oil. Palm oil is cheaper, but a palm isn't really a tree, and tropical conditions ensure high productivity. Probably your alder will only add up if you also crop the wood for some purpose. What can alder wood be used for?

PFAF says Alnus glutinosa is a potential wood-fuel crop in suitable conditions, but gives it an edibility rating of 0. It says Alnus incana is a suitable wood-working crop (but what price does it get in comparison to, say, ash or beech or oak or birch?) and has an edibility rating of 0.

Maybe you can discover this edibility rating is wrong, but you have an uphill battle. I know alder have substantial "cones" (but not the kind of cones that conifers have, really a fruit). In fact looking down various alder species on PFAF I find no one bothering to try eating the seeds. I find native americans eating the catkins of Alnus rubra. Perhaps the seeds inside the cones are really small and not worth the bother. Perhaps this is all a bit like proposing to grow carrots or cabbages in order to eat the seeds.

The obvious example of a British native tree which is used as a seed crop is hazel, you have thought of that already.

A foreign tree which grows well in Britain, thrives on moist acid soils, and was used a seed crop by the natives is Monkey puzzle, Araucaria araucana. It has seeds that are the size of chestnuts. It is also excellent timber, which is why there is so little left in native areas. But it is very slow growing, which is why it isn' t being replanted. The related Araucaria angustifolia is used similarly and grows much faster, but is probably only hardy in the mildest corners of Britain, being from Brazil. Since it is quite uncommon for people to grow groves of monkey puzzles in Britain, which is what you need to get any seed, I have no idea whether British conditions produce good seed or not. The seed cases are very tough. Typically you have to boil them in a pressure cooker for a period to cook them and soften the cases to get them off. I tried this without a pressure cooker, cooked them for 4 hours, and found getting the cases off an awful job, I shredded my hands. The product was rather like chestnut. The trees are dioecious (male and female), and take at least 30 years to come to maturity. Even in Chile, pinones are a novelty crop today, no longer the staple diet of Puenche.

Another major tree seed crop from climates comparable to ours is Pinus koraiensis, Korean pine. This tree is the main producer of pine-nuts in the world. It grows not just in Korea but eastern russia, up to 2600m, so it is very hardy. It grows in poor, acid mountain soils. But it needs it to be very well-drained. I'm trying to grow one on the garden, hoping to get some nuts one day on the suggestion that it is partially self-fertile. It is now about 7 years old and so far all the cones have fallen off before ripening. But mine is growing very slowly, but then I'm growing a decorative garden cultivar which grows much more slowly than the species.