I'll fly out of Seattle at 11:50 AM on April 25th and arrive in Detroit at 6:58 PM.
I'll fly out of Detroit at 8:15 PM on April 29th and arrive in Seattle at 10:11 PM.
I'll pick up and return the car at the airport in Detroit.
The hotel is in Pontiac along with the convention.
The convention doesn't start until 3 PM on the 26th and ends at 7 PM on the 28th.
This will give me, at least, half a day on either end for other activities.