On a theoritical level, yes the tyre should have the correct speed rating as the manual says, and yes this may be used by the insurer as an excuse for non-payment. And yes, the dealer should have fitted the correct tyre.
On a practical level, I am not too concerned about the fact that you have a 2.5L tyre fitted on your 3.0L car, even if it is technically a no-no.... you are not likely to go anywhere near 149mph anyway. But what would worry me more however, is the fact that you now have two different tyres with different treads and frictions on the same axle. This is NOT illegal (the only legal requirement is that both tyres on same axle must be of the same type e.g. steel-belted radials), but in my opinion a far greater problem than the slightly low speed-rating.
This can cause the car to brake unevenly, especially in the wet, and will not do any good to your long-term suspension wear.
Another common problem is when the two front tyres are identical to each other but are not the same make and model as the rears. This is often overlooked, but if the tyres' perfromance is different enough then you could find your tail overtaking your front end when braking in the wet, even though all four tyres are brand new and made by well-known high-quality manufacturers.
Both my Omega always had their tyres replaced in complete sets of 4.