By 1450, neither multibarrel guns nor elevation mechanisms needed inventing. THe Burgundian artillery, then technology leader, used 'ribaudequins', small-caliber multibarrel breechloaders, for antipersonnel applications. Its medium-caliber guns - both the more common breechloading and the less usual muzzleloading designs - had ean elevation mechanism that opearted by hinging the bed of the gun to the carriage. This was impractical for larger calibers, but then, they used those only for sieges, and castles don't shift around much.
Pictures from 15th-century manuscripts also show a number of other designs that may have been used, or may just be imaginary. We know that swivelguns mounted on carts were occasionally used, but I doubt they were very common. A Swiss manuscript show a battle between boats covered with wood armour, with small-caliber guns protruding from those turtle shells. A few designs featured rotating gun platforms that always have the ready gun facing the enemy while those facing back are serviced and loaded. Leonardo tied into a common theme of his day with his weaponry designs, he just drew them better. What I find really interesting is his attention to mechanical details (but even that is not all that uncommon).
Oh, and of course he could have invented the steam engine. Anyone could have at the time. Maybe he did (we still don't know how some of his alleged automata worked). The earliest modern design is by Giovanni Branca, made in 1629 and highly impractical. What would he have wanted it for, though?
