Yes. Grapeshot can be fired by any cannon (i.e., any weapon that fires using the 'Cannon Fire' special rule), as described on page 226 of the Warhammer: The Old World rulebook.

Yes. The Engineer Commander doesn't fire the steam cannon, imagine there's a (very hot and bothered) crew hidden away inside the Steam Tank whose endeavours leave the Engineer Commander free to fire their own weapon.

No. The Steam Power special rule can be used by a Steam Tanks, a model that cannot march, instead of marching. In other words, in instances where most models or units would increase their movement by marching, a Steam Tank can instead make use of the Steam Power special rule. Charging is not such an instance.

No, only unsaved wounds caused by the bearer (the rider) count.

Yes. As stated in the item's rules, a unit carrying the Griffon Standard can claim a Rank Bonus of +2 for each extra rank behind the first, rather than the usual +1, meaning a unit with two extra ranks could claim a Rank Bonus of +4, rather than the usual +2.