My understanding was that it is solar return, i.e. current planetary positions when Sun is at the same position as at birth. Yes, true, but also you need to use a location to determine the lagna or ascendant at the time of the return. How much weightage should be given to solar return chart over say the current Vimshottari Dasha period from Rasi chart. I have never used Varshaphala, so I couldn't tell you from personal experience. I feel that is correct.
Reality is complex.
They are all functioning but in different parts of the realtiy, of the life, and it is up to us to figure out where. I cannot tell you exactly the answer to this question, but I can say that they all work. No one of us gets it all because it is bigger than us. So, you have to study real charts gradually with all these things and find what works, or take lessons from those who use them to hurry up your learning. I myself use only the Rasi, the Vargas, the Dashas, and transits.