with hairy flowers? I think not

This is a plant known as 'Espostoa Hybrid'; google it, I can't provide you with more info.

apart from the fact that the second plant is a 'cutting' from the first plant, the first one doens't look like S. marginatus; the ribs are way too sharp. cereus peruvianus, I agree

definitely not Browningia (they don't have hair).
Pilosocereus gounellei ssp. zehntneri?
could be something else though, it's too small to say for sure.

yep, Tony's is spot on I think

Yea, it sounds like you should be fine. One note of warning though: I've sown quite some Pilosocereus species, and I've noticed that a lot depends on the quality of the seed (and the seller, where did you get yours?). I either get close to 100% germination, or 0%; almost never an 'in between'. What ...

yes, I'd go with that as well.

1) A cleistocactus?
2) Oreocereus/Pilosocereus
3) monstrose cereus
4) same species, yes
5) Mammillaria elongata
6) a Cumulopuntia that has somewhat etiolated in the past? Give it full sun, they're tough.
7) yes

I second Dan

any chance for pictures, if not via this site maybe a photobucket link?

I'm somewhat baffled by what you're describing

yes. Everyone that tells you to give them air once they sprouted (to prevent rotting) didn't sterilise their soil...
I keep them in bags for a couple months, up to a year.

I have to agree with Ian here. The 'classic' baggy-method for starting seeds works perfectly fine, if you do it right you get 50-100% germination rate, easily ('difficult' species excluded). The reason this works is because you have a very high level of control over the germination environment, allo...

I'd say Disocactus (Aporocactus) flagelliformis for the last one, mine also has the tendency to look etiolated even when it get's a lot of sun.

quite probably,
Pilosocereus lanuginosus

3) Pilosocereus lanuginosus

or it's a graft and both rootstock and graft are flowering (as unlikely as the next option )