Triple turrets could improve Mikasa but an aft-heavy layout remains awkward

Would triple turrets have made Mikasa more efficient and a battleship-cruiser fusion more plausible?

Potentially yes, although the proposed aft-heavy arrangement would be undesirable.

Contemporary technology could make triple turrets improve the design and perhaps support a battleship-cruiser fusion. The reservation is tactical layout, with main guns forward preferred over an arrangement weighted toward the stern.