Hi Sekhmet,
Not surprisingly … I would agree with this as well. While a 13" latex mattress will 'work" … it won’t work “as well” and you may find that the cover bunches up more as well when you raise the head and foot of the mattress.
There is more about the differences between Talalay and Dunlop in post #7 here. The main reason to choose 100% natural Dunlop vs Talalay (or the other way around) for the bottom layers would be your own personal preferences and if you don’t have a compelling reason to choose one over the other or if you are unsure I would stick with their suggestions.
You are certainly looking at a great quality/value choice and assuming that you pull the trigger tonight … congratulations on your new mattress 
I’m looking forward to your comments and feedback when you receive it.
Phoenix