I think part of the problem is you are giving the PBX tech/programmer too much information. As a group of people who deal with wiring and phones and whatnot we are all conditioned to look at a wiring diagram and try to then figure out how we can do all that. What I have found is that not all installers are created equal and sometimes all they really need are the basics.
Something like this:
Simple View of Dialer<->PBX Integration We can argue over my naming conventions (or lack thereof) later, but i'm sure every installer on here who has programmed multi-T1 PBX's would now understand exactly where we need to end up. Now the question just becomes "How can we do it".
Maybe the system cant have a blended T1 because of a programming limitation, or have an outbound trunk pool on two different TIE lines. Who knows, but you will get to that stage quicker.
What's behind the PBX should not be much of your concern, the vendor should know his backyard and be able to tell the customer what extension the TIE lines will be on. What's in your backyard also has nothing to do with the PBX Vendor, he could care less that you have an IP Port and 50 computers hooked to it.
The big-picture diagram that you already have is fine from a system-wide perspective. It will look great in a sales pitch where someone thinks they are buying goverment secrets. All the vendor really needs to know is how he'll be connecting, not really what is going on behind it.
The thing to remember is you are doing an integration with them. That PBX was there first, and unfortunately, you are going to have to give more then the Vendor. What that means is if it's a lackluster vendor who doesn't know he may say it cant be done instead. Probably the best advice I can give you here is make friends on this board, they are invaluable.
