#2
no you cannot use a bass in a guitar amp. but you can use a guitar in a bass amp. yes i know it is dumb but a bass's low note will mess up a guitar amp
#4
Bass amp = made for those low frequences that the bass guitar gets

bass in a guitar amp = bad idea as you might end up damaging the speaker (sometimes you can get away with it at bedroom volumes but its still not the greatest idea)
guitar in a bass is fine though.
#5
Yea course you can use bass in a guitar amp, its still an amp

As long as you arent cranking the volume and bottoming out the speaker (it'll make farting noises) Its fine to do that, i have used my little 30w guitar amp to play bass along with other guitars. Not at drum volume though, i doubt you'll get enough lows from a guitar speaker.

And yes, guitars do sound good through a decent bass amp