Woltz, the novel states that Michael must have been scared, but yet he was calm enough to ignore Clemenza's instructions to come out blasting because he reasoned that that was not appropriate to the situation.
I think that if the gun wasn't there, Michael would have continued his conversation with Sollozzo, feigned agreement, and then the Corleones would have tried to arrange another meeting or just gone after Sollozzo otherwise.
I agree with this. Michael would not have tried some desperate backup plan - he would have bided his time, earning a little trust from Sollozzo in the process.
Setting up another meeting would not be a given (as you allude to) but it would be worth a shot, with alternative plans being devised as well. Though I would expect another hit attempt on Vito if they waited too long. Michael's not the only one who could act one way while plotting to go in a totally different direction.
