Sollozzo was piling on the flattery on Vito and Tom too
Also smug the Corleones hadn't found out Barzini's connection that it was Barzini all along

I believe Vito's million was insurance ie: Vito's own money invested - for Vito's commitment and guarantee that he wouldn't change his mind and pull the plug on his protection
I think everyone would have had to invest some money of their own

Whilst I appreciate Sollozzo may have been “very, very scared he botched” Vito's murder, did Sollozzo really think Sollozzo could string the Corleones along until third time lucky! even without Luca -
1. killing Vito
2. kidnapping Tom to persuade Sonny to make peace as if...
3. threatening Tom on finding out Vito had survived
4. meeting with civilian Michael to buy time

Then again if not for Michael....