These two "dang" should be understood as "acting" or "replacing" in poetry. In addition, "ke" does not mean "neng" in modern Chinese. "Ke" means "can" and "Yi" means "use". These two poems are about the sadness of a wanderer who is homesick and can't go back.
"Sad songs can be used as tears" means that wanderers miss their hometown, but they can't burst into tears, so they have to use sad songs as tears;
"I can go back to my hometown in the distance" means that the wanderer looks forward to it day and night, but he can't go back to his hometown, so he has to take his hometown as his hometown.
Then why doesn't he go back? Nobody wants to go home. Wandering outside, like rootless duckweed, always looking forward to returning to the hometown where the soul can get peace, but without relatives at home, even if you return to your hometown, you are like an outsider and a guest.