I would hold off buying acrylic for the moment until the 90º v 100º issue has been fully finalised, I don't know what general timescale you envison for your brother's visit?
Can we take it that you have at some point built a scanner with a 100º platen, and if so did you use acrylic or glass?
Looking at my drawing above of the end view of 90º and 100º platens, do you have any comment on the end view for the 100º platen, which is not as neat as that for the 90º platen? The fact that the two sheets don't butt together neatly might slightly complicate assembly, although it shouldn't be a barrier. And also from the point of view of easy insertion into an open book?
Like you I intuitively feel that the two pieces of acrylic would be better joined along the length: I can imagine them possibly separating slightly when the platen is raised, and being more likely to deflect as the platen angle is increased to 100º. On the other hand, they could tend to be forced together as the platen is inserted into the book, and in David Landin's final baseline design they were not joined.
With regard to joining them, David Landin makes the following comment which could be relevant to a 100º platen:
You need very little of this cement, and you do need a very crisp, clean edge to the plastic, so that the edge of the plastic make an excellent contact with the sheet it is being glued on to.
You or I could probably cut the two edges that join at a slight angle so as to get good contact, but I don't see that as being a practical option generally. And if linking the two pieces is worthwhile, then I would think that just joining them securely at spots say 2" apart would probably be sufficient, and safer than trying to join them along the whole length. Maybe a general purpose adhesive could be used that could be pressed into the slight space between 100º sheets?