#4
If the head has two speaker outs just connect one to the first cab then buy a second cab and connect it to that.
#5
Having a full stack won't make you sound any better. A meh high wattage solid state amp going through even more low grade speakers won't improve the sound quality, and probably won't sound any louder.

By the time you got done with buying another speaker cabinet you could've gotten a decent tube combo that doesn't require roadies to lug around, and of course it will sound much better