The warning system in the Mercedes C-Class in that test was a different light from the ones we have in our cars here. Notice that the C-Class had scored a 4-star rating in the test just as all of the other cars in that class did, but because of the addition of another light that said "put on your seatbelt", the car's score was later changed to a 5 to promote more lights that say "put on your seatbelt" to be used in other cars. I'm not saying that the C does not deserve a high rating in crash tests, but its additional star over the other cars in its class is simply because it has an additional light that tells passengers to wear their seatbelts on top of the light on the dash that says to wear your seatbelt. It is not necessarily safer than the other cars because of that light, and so I do not believe that it should have gotten an entire extra star for a light.
You are also wrong about the 3.0 Jaguar engine "pushing" into the cabin in a crash before the 2.5 and 2.0 because they are the same engine blocks but the 3.0 has been drilled-out more to be larger. Also, the engines do not enter the cabins at any time in a crash because the X-TYPE is designed to "drop" its engine when it is in a severe frontal crash. Most modern cars do this. X-TYPE's engines are also mounted horizonatally, so they are not taking up a terrible amount of longitudal space in the engine bay, thus the chances of them penetrating the passenger cabin is slim in comparison to longitudally oriented engines. Again, however, the X-TYPE drops its engine when it has a severe frontal crash, so it never enters the passenger compartment no matter what engine size is used. The argument that a 2.0 FWD X-TYPE would be safer than an AWD larger displacement version is null and void, because the engine does not enter the cabin at any time during any impact.
You really should read the bottom chart of the EuroNCAP page for the X-TYPE:
It specifically states that the car is: "Jaguar X-Type 2.0 LHD. Only the FWD X-TYPE is built with the 2.0-litre engine, which again is the same engine block as the 2.5 and 3.0, but not drilled-out as much. All other X-TYPEs are built in AWD configuration. The AWD driveshaft of the AWD X-TYPEs aid in collisions v. the FWD version that is sold alongside them in Europe against 318 and C180.
Mercedes-Benz decided to changed to a V6 from an I6 because they lowered the bonnet of the E-Class between those configurations and could not fit the I-6 under the lowered bonnet, as well as because it may have cost less to produce and it could be used in more vahicles as it had better packaging. That is also the reason for the discontinuation of the E300 diesel, as the diesel engine could not fit under the lowered bonnet that was used when the V6 was introduced (that and because the E300 did not sell well here). The E-Class should have been designed to drop its engine in a crash as well. I've not heard of a modern car being designed so that its engine penetrates the passenger compartment in a collision.