Originally Posted by Turnbull
I think the reason is that Sol knew Tom favored the deal even before the shooting, and said he'd "try" to persuade Sonny and the capos after Sol kidnapped him and let him go. But obviously Tom was not successful in getting Sonny to come around. Why try Tom a second time? Perhaps Sol thought that Michael, as a "real" son to Vito and a "real" brother to Sonny, could be more persuasive.

"Personal, not business."
When Sollozzo made his [first] proposition - make peace between Sollozzo and Sonny, persuade Sonny to make the narcotics deal - to kidnapped Consiglieri Tom, Sollozzo believed Vito was dead

I think Sollozzo realised, with Vito alive, Sonny won't / can't make the deal even if “Sonny was hot for his deal”

Sollozzo's “Well that's bad luck for me and bad luck for you if you don't make that deal!” was a knee-jerk reaction out of frustration and trepidation, having to explain his failure to kill Vito, to Tattaglia [Barzini] and start all over again

I believe that's why Sollozzo requested the meeting for his [second] proposition, buying time until Sollozzo could make another, this time successful, attempt on Vito's life

Sollozzo didn't know that the Corleones had a man in McCluskey's precinct and would find out the 'secret' meeting place
Sollozzo was smug, secure and invulnerable being guarded by the Police captain

In my view, whatever Sollozzo hoped to achieve with 'civilian' Michael which I believe was to pass on to Sonny, Sollozzo's revised second proposition - his insincere truce request, Sollozzo could have, with Consiglieri Tom as either was just the 'messenger'
And if it was Tom! Sollozzo would have lived!!

My understanding is, Sollozzo wanted the Corleones to interfere no longer until Vito has recovered to start discussing the narcotics deal again or similar - his insincere truce request – not because “Perhaps Sol thought Michael, as a "real" son to Vito and a "real" brother to Sonny, could be more persuasive” to go for the narcotics deal?

Besides in fairness to Tom, he didn't get the opportunity “in getting Sonny to come around” because Vito was not dead