Here's my 2 cents for what it's worth!
The only three points engaging the cylinder are:
1. The cylinder stop
2. The cylinder pin on the ejector rod
3. the cylinder pawl to rotate as the hammer is pulled back
Since the cylinder latch moves back and forth, we might assume that the latch is pressing in on the cylinder pin to release it.
So the question for everyone to consider would be what else could cause the cylinder to not open?
The OP has already stated that there is a cylinder gap, so a squibb or bullet between the barrel and cylinder could not be the problem.
A high primer dragging on the rear?
Check with a flashlight to see that the pawl is not engaged, but since the hammer is down, I would assume that the pawl is not.
Someone suggested that the ejector rod might have unscrewed, but couldn't that be solved by looking into the barrel shroud to see if the rod is binding by being unscrewed?
So if there is no bullet in the gap, no pawl engaged, and the cylinder latch pushes in, why not use a rubber mallet to force the cylinder open?
That would be my line of thinking if it was my problem to deal with.