Yes. Use Grab, a metered Mai Linh or Vinasun taxi, or a driver you booked in advance. Avoid unmarked cars offering rides at the river, and have your hotel address written in Vietnamese.
Yes, for street food, boats and the market. Cafes, hotels and tailors increasingly take cards. ATMs are on Tran Hung Dao street; choose the dong amount, not the conversion.
On the 14th day of every lunar month. The old town switches off electric lights and floats candles on the river from 18:00. It is the busiest night of the month, so book boats and dinners early.