I'm not sure the MGB front suspension would have been any better in terms of refinement. It after all dates from about the same time - but Austin based rather than Morris. And still used that abortion - lever arm dampers. The SD1 and Dolomite range had already got reasonable live axle rear suspension - one could only assume it was too expensive or possibly too intrusive in boot space for the Marina.
The Maxi was actually quite a decent car let down by the usual lack of development before sale and dreadful build quality.
Perhaps the worst feature of the Marina was that even the bits it did use weren't properly sorted at design stage - early road testers complained bitterly about the understeer and forced BL to do something about it. Many also suffered extreme clutch judder. Given the a bog standard layout such things are unbelievable.