Hi Andy, if you have the Diamond pass you are under no obligation to use all of activities included in the ticket, if there's a part of it you can't or don't want to do you don't have to, but you cannot tailor the pass to your wants. It's either the whole package or not, you cannot get refunded on that portion of the ticket you don't use.
If there is an additional talk on Sunday you would have to buy an additional ticket for it as that talk wouldn't be included in the Diamond package and individual aspects of the ticket aren't usually transferable. As to if there is one taking place on Sunday I don't know, there aren't tickets on sale for one yet so there may not be.
People will tell you that the only way to guarantee an auto is to have a Diamond pass. Beyond that we can only speculate. He is a rare guest so a lot of people will want him. However other than MJF who was ONLY signing for Diamond pass holders I don't think there has been a guest who hasn't reached the VQ holders but they may not get through many. In 2015 Sigourney Weaver only got through about 10 non Diamond pass holders but beyond that I think most D Pass guests have done a few VQ ticket holders. It's a smoother process now that most people know how it works. I think Jeremy Renner got through a lot of VQ ticket holders last year and people weren't rushing through at light speed. Don't forget you have the Gold Pass holders to contend with if you want to consider going for a VQ ticket as well and they get into the venue 10 minutes before general ticket holder. Also how many days the guest is attending factors in, the fewer days they are there the less people they will get through.
What it boils down to is though is that no one can really say how it will go as there are so many factors involved, not least how quick he is at signing. If he's slow he's going to get through fewer people. If hundreds attend just for him there's going to be less chance.
You may be ok without the Diamond pass, a lot of people were last year, but no one can really say. If you want to be 100% certain of getting it, and you really really want to get his auto, and can afford the pass I would prefer to get it myself. That way you know you are sorted. If you are traveling down on the Sunday it's good to remember that people do start queuing very early especially when they have big/rare guests as they will want to get VQ tickets themselves so depending on when you arrive at the venue you may have none or a lot of people queuing already in front of you.
Remember to buy an entry ticket as well though.
The only other option would be to get one through the pre order service and you can book a photo shoot with him for Sunday so you actually get to meet him.
Thanks very much for the very useful advice.
Going to check the funds and make a decision :-)