All the data the Excalibans had about Kahless came form Kirk and Spock's memories. Not exactly unbiased sources.
Correct. Then it was Federation propaganda at the expense of the Klingons?
@ Edit XYZ
While Buddhism is practised as a religion in Eastern Asia I recall reading that when the Buddha passed away he asked that his teachings are remembered and cherished, not his person.
And supposedly the Dalai Lama once said "We are not here to believe but to learn". This doesn't strike me as religion but rather a "constitutionalized" philosophy / spirituality.