Dragons have different wings to birds and bats. Their wings are attached near the front of their body. Birds and bats on the other hand have wings attached in the middle. That means the dragon needs less air to begin flying.
So use the wings like a bat to launch into a rampant position (see Somerset Flag)
Then push with the back legs for a ...
How about making the forelegs very long and thin, not strong but with spring like tendons?
It creates another problem when figuring out how they tuck away in flight.
But maybe it justifies how they take food to their mouths and such.
Very slowly and probably not at all
Any evolution of the kind you suggest would have to happen over a long period of time. In reality, it's really unlikely that this could ever happen.
But since this isn't Biology SE, let's come up with a plausible way it theoretically could happen. You could have an aquatic create evolve an air bladder to control its ...