True Michael was unwilling to rely on anyone else to kill Sol and Mac because it was the only foolproof way.

I will have to watch that scene again but as I remember, it was short notice
They were panicking that they didn't know where the meeting was going to be when they were eating dinner

Suddenly if there is a lot of activity at and around the restaurant and more than usual men dining there even for the best veal in the city, I reckon it would attract unwanted attention

Also it is fleshed out in the novel, Sol had his people check the restaurant out, bathroom and all and a man was actually dining there, keeping watch

As regards a frontal assault, Sol could have had people guarding the restaurant too