Questions regarding で, に

Question

ロビーが満員で入室できませんでした

悩み事で眠れない

Does this で mean "because"? If so, can it be replaced by だから、なので and/or だということで? (I think that で in the first sentence is supposed to be だ/です, but can it be translated as "because"?)

I think that に can also mean "because" at times, would it work this time?

ロビーが満員に入室できませんでした

悩み事に眠れない

Regarding the second sentence, is the で used the same (だ/です)?

As you can see, I'm a bit confused with all these way of saying the same thing.

Answer 1

で can be used to represent causal relationships in a similar way to some english uses of because, although I am hesitant to say that で=because. Your two uses of に do not really work; I would stay away from trying to use に as "because" in most cases.

Let's talk about で though. You've actually picked two very good examples, because although the で in each of them is representing a causal relationship, they do so in different ways.

ロビーが満員で入室できませんでした

This で is an abbreviated であり. You can read more about this here, but this is very close to an English and, and it can be used to represent causal relationships in a similar way.

ロビーが満員であり、入室できませんでした

The lobby was full (of people) and I could not get in.

No because necessary. Also note that ロビー is lobby, not room.

As for

悩み事で眠れない

This is the で particle we know and love. 悩み事 is the thing directly facilitating somebody not sleeping, in the way that the plane in 飛行機で行く directly facilitates your traveling.

I think it's safe to translate this as "I could not sleep because of my worries", but I encourage you to think of this more as a natural extension of the way で is used than as it corresponding directly to because.

Edit: It might be best to think of because of, rather than just because, as the possible translation for で here. It really only plays this role with nouns, which in English will end up as because of [noun].