These are all very good points, and shouldn't be lost on those wanting barrel conversions for other systems. Dare we mention a well-known non-restricted platform imported recently?
With that being said, one case where it does NOT apply, is the conversion of a restricted handgun to a barrel length <106mm. In this case, the law says that the short barrel - e.g. a factory barrel for a G19 or G26 - is a prohibited device even if it's just sitting in your sock drawer. Much like a normal-capacity AR15 mag is prohibited even if it's unloaded and not in the firearm. Even so, the 'conversion' law may actually apply - but the charge would most definitely be "possession of a prohibited device" - against which there would be no reasonable argument under the current laws.
Other than that, seems that the barrel swap is a "go" provided one makes their best effort to obey the laws regarding where each barrel length can be used.
-M