Of course, Scott/Webley is known for it's contradictions and exceptions so the charts are simply a basic guideline. For example it is not uncommon to find "Proprietary" model (A&D type Model 400, etc similar to more modern 700 series) Webley boxlocks with extra bells/whistles and the W&C Scott and sons stamps on them. After all, it was one big manufacturing company after the merger and "parts is parts".
Not really. Guns bearing the Scott name were assigned numbers from the Scott blocks, and those bearing the Webley & Scott name were assigned numbers from the Webley blocks.
As for mixing models, I can think of one model Webley that was sometimes sold under the Scott name, but this was generally not done.
