I believe “a plane dispatched from the US” undoubtedly would have attracted attention

Roth knew the $2 million never got to the island and then Fredo had brought a bag full of money so “a plane dispatched from the US” would have been flashing neon lights!

Whilst I believe the private plane whether Cuban or American would have waited and waited, standing by for their passengers - the Cuban pilot may not even know who his passenger/s are [believe the plane arranged by the Hotel Concierge] Michael still has to get to the airport

How Michael planned to dodge? give the slip? to the "team of assassins" who would have been “lurking inconspicuously at the Presidential party to make sure Michael didn't run out on them or change his plans” to “carry out their assignment and murder him”

So, brings us back to, why Michael needed
1. to attend the Presidential party
2. an alibi Whom did Michael have to answer to?