[MLD-Grounds-Keeper-Willie]

Sam has every right not to trust Gollum, but it wasn't mistrust that prevented Smeagol from staying. No doubt that first passage is the key struggle, but turning point (for me) is

'Hey you!' he said roughly. 'What are you up to?'
'Nothing, nothing,' said Gollum softly. 'Nice Master!'
'I daresay,' said Sam. 'But where have you been to - sneaking off and sneaking back, you old villain?'

Sam doesn't have to trust Gollum, but he doesn't have to be such a bully either. After Sam's first interogating question, Smeagol is still there and is being kind and even understanding to Sam, who asks "roughly." Then Sam chooses to say "sneaking" instead of something like going off. And he just has to throw in an insult ("you old villian"), which is where Smeagol leaves for good.

Because Sam doesn't like Gollum, he constantly picks on him and bullies him. He judges Gollum on what he has done, in another life. Gollum has no doubt suffered enough to make up for his evil deeds that were mainly because of the ring anyways. And biw he's ready to repent and give up the ring in exchange for friends to finally end his loneliness and suffering. You are not to judge a released convict because of what he has done before in another time to someone else. When that convict is out of jail, he has served his time and is willing to start his life over. It's not fair to judge him like that if he has done nothing to him. Sam, however,

Would he have told them of her had Sam not woken thus?

Well, to answer that:

peering back up towards the pass, shaking his head, as if engaged in some interior debate.

I say yes, he would have. It seems that he was debating whether or not to repent and not lead them to Shelob. And it looked like he wasn't going to lead them to her when he touched Frodo's knee and it looked like he just wanted to give up the ring. But I believe that he chose to lead them to Shelob as a result of Sam's wrong, cruel actions.

but there was so much in Gollum to forgive

Yes, but that is not for Sam to forgive. The killing of Deagol was not something harmful in anyway towards Sam, therefore it is not something that is up to Sam to forgive.

but we can hardly affix a great deal of blame to Sam

An addict battling temptation and addiction is a fragile thing. Push them just a little and they will fall. Sam pushed Smeagol. I place the (great) majority of fault on Sam, for Sam was a bully to Smeagol.

I hope I made myself clear, if not, I'll try again.