How big can a but joint be in face brick on wood studs? How much of a variance can there be in but joint sizes with tumbled brick?

I believe I am seeing two completely different questions. one is in regard to the material that will used as the substrate in your wall assembly. You will need to contact your local building department to solve the issue of fire-treated vs non combustible plywood. As far as staples in gypsum board, that method should not be used. Adhered veneer is supported by the substrate it is applied to thru the lath and the anchors used to attach the lath. I am attaching a couple of items that will give you direction on the assembly.

One item to read is ASTM C1780 Standard Practice for Installation for Adhered Manufactured Stone Masonry Veneer. Even though this Practice does not include thin brick, the substrate preparation is relatively the same. Another document is BIA Tech Note 28c for Thin Brick Veneers. It is an excellent reference document. If you know the brick and its manufacturer additional information technical information is generally available on their website.
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I hope this is helpful in designing and constructing a beautiful and functional thin brick.
Jerry Painter, FASTM

About eight months ago, you helped us getting some bond beam clarifications on a project we’re doing with PCL construction (see original email below). Recently we’ve encountered another bond beam issue on this project that we’re having a difficult time overcoming. The bond beam details show it located in the last course as seen it in typical applications (see attachment). Unfortunately, all of the overhead decks are installed and placed, making the installation of the bond beam on the top course very difficult to install and grout. We proposed to lower the bond beams by one course but we’re getting resistance from the engineer. PCL asked us to reach out to you in hopes that you could help shed some light into this and ease the engineers concerns that there are no structural concerns by doing this. Is this something you could take a look into and help us out with? ​

The designer was hopefully thinking that the walls would be built first before creating this detail. As you are well aware the installation of the block is not the problem although a 3/4" space at the top would be more realistic and wouldn't effect the structural support of the wall by the very healthy 4x4x3/8 angles. If the engineer felt the need you could easily increase the number of anchors on each side from 2 to 3 to compensate for the additional moment from the wall.

But that only addresses the installation of the masonry unit itself. The next problem is the installation of the grout which cannot be squirted into a 1/2" crack at the top of the wall. A common way to grout masonry walls under an existing slab is to core a 3" or 4" hole in the slab at 4' or so along the wall and grout the wall through those holes. It does sound like the engineer would like the top course grouted to increase the lateral load resistance. Failing being able to grout the top course through the cover slab you could engage some exotic method of porting and pumping a very fine grout mixture (it wouldn't meet C-476 but would work) into the wall at a lower level.

The vertical steel is the easiest and you have probably done this many times. Just create a hand hold in the wall and slide a lapped bar up into the top course after it is laid in place.

The horz steel as detailed is virtually impossible to install as you already know. This steel can only serve two purposes: 1) to span the 6' 8" laterally between the 4x4 side plants and 2) to provide horz reinforcement for crack control. The crack control reinforcement would actually perform better in the next course down ( or 3 or 4 courses down - makes no difference). Concern about the wall spanning laterally 6' 8" could be addressed by simply reducing the steel size to a #3 bar which could be slid into a hand hole in the top course. Another solution would be to distribute your latter supporting 4x4 angles down the length of the wall rather and a single 18" long piece at 8' o/c. Say... something like a 4x4x4" lg piece at 2' o/c. Your lateral bending between the supports then becomes insignificant and your (2) #5 bars in the second course down would take care of your crack control slightly more efficiently then if they were in the top course.

Other than that you can form and pour the top course which still doesn't address how you get the concrete into it.

I am honestly surprised that the engineer of record would not immediately recognize that he had created an extremely difficult (impossible) detail to construct and be helpful in rectifying the situation. If it has not already been done I would recommend providing him with the opportunity to come up with a reasonable engineering solution to the problem. Perhaps he has not been informed of the problem?
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Links to additional information:

CMU Wall Details: https://bit.ly/2GZRWhC​ 

​What is the contractor's responsibility in specifying the strength of concrete?  For example, would 2,500 PSI High Strength achieve the wind loads and 20ft wall heights?  Or would 3,000 PSI CMU be needed for 20’ walls grouted 8” o.c.?

That is why you hire a structural engineer!  It depends... on the location, height of the wall section, vertical load, fixity of end conditions, openings, lateral support, etc. You're not designing the wall section, simply offering advice on the availability and cost of materials along with potential problems with the use of various materials. Without designing the wall section myself - which would include understanding the entire structure that the wall is a part of - it isn't possible to challenge or advise him on the design.