In deaf mourning [w głuchej żałobie]?
That one is gramatically correct. One can say "w głuchej ciszy", so why not say "w głuchej żałobie" - in mourning so deep that you can't hear anything else beside your deep mourning?
In this one he probably meant "w mrocznych czasach", but since we are talking about the mourning, the deaths, the graves, you cannot guarantee that it is not some old epitaph where you can find the word 'czasze' nicely coinciding with the word 'czaszki' (skulls).
Let us hurry to love people they depart do quickly.
This is a cliché that has really become too banal in Poland these days (IMHO).