The working conditions are unhealthy, if not downright dangerous.
Does it mean
The working conditions are unhealthy, and probably extremely dangerous."
The working conditions are unhealthy, if not downright dangerous.
Does it mean
The working conditions are unhealthy, and probably extremely dangerous."
That's right.
In a disapproving context, "X if not Y", means "X and quite possibly Y, (which is worse)"
You are partially correct. However, ... is X, if not Y is an idiom, and there is an additional meaning that is not obvious. In your example which is a negative situation, Y is a worse thing than X.
Another example.
This is untrue, if not a deliberate lie
This means something like: "It is untrue, but it's probably worse and is in fact a deliberate lie".
A similar way to say this is
This is untrue; at worst, [it is] a deliberate lie.
Note: For this idiom to work in negative situations, Y is usually a worse thing than X. However, you can also use it in a positive way, e.g., "... is good, if not superb", where Y is better than X.