Na Hee Do and her mom were in the hospital together because they went in for colonoscopies at the same time.
They didn't show Na Hee Do's husband because the story isn't about that.
I just finished the show, but I think the tunnel scene was just to show what they each said to each other in those last notes. The words that they really wanted to say (instead of the nastiness that they spewed at each other earlier). Na Hee Do wrote it in her diary, but then lost it. Baek Yi Jin read what she wrote, and then added his piece to her diary, but then it didn't get back to her for YEARS.