If we take the bus back from Tina’s guesthouse to Lijiang.

I'm afraid you cannot because the bus leaving Tina's Guesthouse to Lijing usually departs at 15:30 pm. The bus ride may needs about 4 -5 hours. So you cannot make it to the Lijiang airport at 18:00. If you want to get to the Lijiang airport from the hotel in one day, you are advised to ask the hotel keeper to help you hire a car, which is the fastest way.