I've read previous posts which stated Merkel shotgun actions are build on the same actions as their double rifles. Has this always been the case?

As far as I know Merkel has always built all rifles, combinations and shortguns on the same actions, with the only difference being size! or o/u, single shot or side by side.

This is a no brainer! It is simply not good business sense to build different actions for double rifles and shotguns. just make the actions adiquately so they work for what ever you use them for.

With the Greener cross bolt, they're very strong actions. So (what Mac said), they work very well.