Nope, they will not let you in if you arrive at approx. 8:45 and later. Have had this happen 2x - they close off the park due to the Tree of Life opening ceremony. The park managers are in charge, and due to safety issues, restaurant patrons are not let in under any circumstances.
If you go about 15 mins. before park opening, you will be held back. I have spoken to park supervisors/managers, as well as restaurant managers, and despite knowing it's a big problem, they refuse to do anything about it. I think they should close ADRs for that time period, as the restaurant gets seriously backed up, as no one is let in during this period.
After the 1st time, they said it was just a 'misunderstanding' but after 2nd time, we realized that there was no misunderstanding. I strongly encourage people to arrive about 25 - 30 mins. before park opening ADR, so that way, they will let you in. It's best to wait at Tusker House, then be trapped at rope drop (we hate rope drops, and have been stuck in them 2x).
We will no longer make ADRS at Tusker House until after park has opened, as it's a big problem with the current system. So, despite what others have said above, they will not let you in if you are about 10-15 mins before park opening. This is the critical time area. Both times, they found our name on the clipboard, but the restaurant CMs were not allowed to let us in under any circumstances, until after the rope drop ceremony was over.
Good luck, Tiger