> 1. Is a valid way of saying "In the dream"?
>
Yes.
> 2. Should that comma be there?
>
It doesn't matter.
> 3. The to say "In a field/meadow full of flowers" part is correct?
>
No. Your sentence is perfectly grammatical but means "flowers were running in flocks". You have to say **** **** . without is an adverb that modifies the next verb (). With this , is parsed as a relative clause that modifies :
*
The field is full of flowers.
*
the field which is full of flowers
And to say "through/across", you need to use before . See: Why and not ?