Here is how I would break it down
雪が霰が降ってきそうに、
I’m not sure why it’s a double が here (could be because 雪 is the subject and が marks 雪 and the later 霰が降ってきそう is just a feature of the snow?), but I would translate it as “it seemed snow was coming down like hail” (resembling hail?). Not quite sure how to put it into proper English .
日の光も当たらずに、
here the に is a part of the 〜ず(に) grammar point which works similar to the 〜ないで grammar point.
So 日の光も当たらずに、 I would translate as “the Sun wasn’t shining either” or “without the Sun shining either”
The full breakdown might make more sense from the end, I guess?
寒うございました。it was/became cold
雪が霰が降ってきそうに、With the snow coming down like hail
日の光も当たらずに、and without the Sun shining either
I think it was explained here when @2OC3aOdKgwSGlxfz had the same question a while back: Help understanding this phrase from the short story 大きなかに
Here is how I would look at it
In the case of 床の中へ 入って I think it’s just a regular “and” clause bridging to connect to the thing after, but without emphasizing these two things are strongly linked. That’s how I would interpret it.
Here the connection is a little more apparent:
ランプの下にすわって起きていたのでした。
Sitting under the lamp & being awake