In the sentence,
C'est hors de question d'aller à la plage par cette pluie !
“par” cette pluie seems more appropriate for “by” the rain, as in, washed away by the rain. “… sous cette pluie” seems like it might be more appropriate here. No?
Also, in the sentence,
Je t'attends depuis longtemps.
Is there something about the verb being followed by “depuis” that requires it to be present tense, rather than imparfait.
So, lit. , I am waiting for you since … , rather than, I've been waiting for you since … ?