When paying for the guide/porter’s tips, food, and lodging, do they take US Dollars/Euros or Nepalese Rupees?
You will need Nepali Rupees for all transactions in Nepal including guide/porter's tips, food, and lodging at teahouses. You can change money pretty easily. There are money changers all over the Thamel area of Kathmandu that have late hours. There are also some teahouses and shops on the trek that will change money but for a far worse rate than you will find in Kathmandu.
More about changing money and ATMs in Nepal can be found on our blog:
ATMs and Changing Money in Nepal