Yes, they can.

No. Because to enter the building it must first make a normal move (no march or reform), and then when it leaves it it'll follow the rules for a unit returning from pursuing (which include making a normal move). If it entered and left the building in the same Movement phase, it would effectively move twice (i.e. it would make two normal moves), which is not allowed.

Well there seem to be only one alternative: simply choose/declare the formation when the unit leaves the building for the first time.

Fire the weapon as normal, determining how many hits it would cause normally. Then roll the D6 for the target being inside the building, and the result is the number of the hits caused by the weapon that actually hit the target. Any excess hits are wasted against the walls.

Their 'fire zone' is as wide as the face of the building they are firing from. The hits are Strength 3, +1 for every storey from which another five blunderbusses can fire.