Sure thing “both hits were in motion BEFORE the meeting”
Sonny's murder was the trigger for Vito to call the Peace meeting and to get Michael back to New York

Sonny's hit was pretty much handed to Barzini on a silver platter after Sonny's public beating of Carlo

I believe the hit on Michael would have been in progress for quite awhile as arranging such a hit in Sicily from New York would not have been as easy as Sonny's hit

Barzini and the other Dons would have had to -

• be very discreet in all their enquiries to ensure no word gets back to the Corleones
• find out where Michael was hiding
• Nobody probably knew until Michael's “I am an American hiding in Sicily My name is Michael Corleone There are people who'd pay a lot of money for that information” to Apollonia's father Vitelli thus revealing his identity
• the expensive gifts and the wedding would have attracted publicity as well

Then all bets are off!

Again it may not have been easy to be able to just call off the murder of Michael from New York [within the communication channels available at that time] after Vito's request for the meeting which was soon after Sonny's murder

I also believe the target was Michael
It seems to me Fabrizio was keen to ensure Michael would be the one to drive that car rigged with the bomb

Fabrizio: “Are you driving yourself, Boss?”
Michael: Yes

Fabrizio: “Is your wife coming with you?”
Michael: “No I want you to take her to her father's house till I know things are safe”

Fabrizio is running away after seeing Apollonia in the driver's seat