The trouble with seeds

Moog thinks the most annoying thing about this allotment business so far is the seeds. How anything grows in the wild apart from weeds is anyone's guess.

The most disappointing seeds so far have been French Marigolds, which I planted to add a bit of colour and fragrance to my plot (they're supposed to keep insects off too). Out of an entire packet of seeds I've got about 10 little plants. That's still good value but not the big border full of colour I was hoping for. The most successful seeds so far are lettuce and courgette seeds. The lettuces are quick and almost all of them come up and start turning into viable plants. The courgettes also produce big healthy looking seedlings, none of the spindly stems that frequently collapse as soon as you water them. Moog thinks they're in proportion to the size of the plant so maybe I just need to grow big things.

I have had the following disasters so far, which have all tested my patience to varying degrees:

1. French bean seeds rotted because they were too cold and wet (despite being on a warm window sill in a propagator). All rotted except one single solitary magic bean which now a foot tall. Replacements coming on fine.
2. Chillis and peppers took ages to germinate. Consequently I managed to separate the seeds from their labels. I was just about to give up on seeds and buy some plants from the garden centre when they all started sprouting. The trouble is, peppers and chillis are identical as seedlings so I now have no idea which plants are which. Luckily, the tomatoes I had put in different shaped pots so I can identify those.
3. Leeks came up well but it has taken me the best part of a weekend to pot them all on into bigger containers, and I have no idea if they will all survive; time will tell. I would like to know how my next door neighbour John has got leeks like sticks of rock while mine are still like blades of grass.
4. When things did come through I was slow potting them on, as I expected them to get a bit bigger first. Moog now tells me I need to pot things on as soon as the first couple of proper leaves come through, otherwise little seed trays/pots get full of roots and the plant stops growing.
5. Some plants germinate but then just fall over dead. I don't know why, I am attributing it to "damping off" according to all the books which say it is due to fungal infection. Seedlings are so fragile, I don't know how wild plants make it on their own.
6. Leggy seedlings have been a feature, as I haven't really got a suitably light-but-not-direct-sunlight place to put them. The kitchen and spare room window sills are too dark, the greenhouse is too hot. Where is the Goldilocks zone for seedlings? Any advice welcome...