Spike's development as a character started as far back as the Season 2 finale, when he helped Buffy save the world from Angel's plans with Alcatha (or whatever it was). His actions may have been based upon selfishness on his part . . . but it was his start on a hard, long road.
His intention was never to start doing true superhero good, though, so I wouldn't call this the start of any road. However I'd buy this if he didn't come back less than a year later to kill again. Then at the beginning of season 4. Then spending a year waiting for a moment to strike. Getting a moment to strike then blowing it.
Then changing, once his hate was so much it turned into feelings for Buffy. If Angel wasn't posing a threat to his relationship with Drusilla I doubt Spike would've had second thoughts and ran to the enemy. He even states this as his motivation. Also, if ending the world meant they got to rule over the dead and own the earth, instead of ceasing to exist like everyone else, things would be much different.
And the chip didn't change Spike's moral compass. Even with the chip, he was willing to help Adam defeat Buffy and the Scoobies. He nearly broke the four friends apart. And he had schemed to get rid of the chip and kill Buffy in early S3. Like I said . . . it was a long, hard road. And he didn't require a cursed soul.
But without the chip his moral compass never would've shifted, which it ultimately did, no? My point was had he not been chipped - which was forced upon him - there'd be no way he would've sought a soul. (Thus both parties had something forced upon them that got them to where they are.) If Angel had never came before him (and had Buffy first) I doubt Spike would've had this idea on his own
especially without a soul. Also, you're right, he didn't have a soul during those moments of doing bad things so I dunno what you mean here.
Do I want Spike to receive the Shanshu? No. Personally, I think the Shanshu is worth crap. What is the big deal about becoming human anyway? It doesn't automatically make one good. And personally, I don't think that humanity's moral compass is that different from demons.
What makes one good is subjective. I don't know why Spike wants it, maybe only because Angel does. Seems in character, considering the bickering and competition of them in season 5 of Angel. For Angel, it means he has repaid the debt he owes for his soulless crimes and is rewarded by something above him to say that it was enough and he can now be with Buffy. I get why he wants it, as it fullfils his superobjective that he had before he even knew of the Shanshu.
I think how the soul was acquired is irrelevent; I think what one does with the soul is what matters. Kinda Biblical: doesn't matter how you come into the world, it's what you do to the world that matters and you shall be rewarded.