Hi:
I have never flown through ONT, but I live in LA and can give you perspective on the rest. You are much better off driving wise going to downtown LA on your arrival rather than waiting until the next morning. You will face significant traffic driving into downtown from Ontario in the morning. On the other hand, you will be driving in the opposite direction of most traffic at 8pm at night. Also, downtown LA is pretty happening with lots of cool hotels, bars, and restaurants. You will pay a premium for this including paying for parking at your hotel. To me, it would be worth it though.
Channel Islands are awesome! Which island are you going to? I've been to all of the openly visitable ones except Santa Barbara Island. My favorite is Santa Rosa followed closely by San Miguel, but all are cool. Make sure you have your boat trip booked asap if you haven't done so already. Like someone else said, make sure you leave plenty of extra time to get there just to be safe.
At LAX, it is possible to take a shuttle to a train that will get you downtown. It won't be fast but neither will shuttling to your rental car facility, waiting in line and driving in traffic. You could then rent a car downtown when you needed it to avoid paying for parking. Consider Turo as a car rental option that might be good for non business hours rentals.
If you have any other questions, I'm happy to help. Btw, I'm guessing Burbank airport is the closest to downtown if they have availability. And, it's supposedly hastle free.