Think I'll leave interlocking for future.
Quite right too! But I think you have demonstrated perfectly why, when developing a 'fictional' layout, it is very helpful to think about the signalling - very well done for that.
Also just to mention in passing that sometimes choices you make in numbering the frame can make life a bit easier for yourself when it comes to the locking. The locking for this layout is pretty simple so there is no big deal, but, as an example, the South Yard entry signal could perhaps be 7 rather than 17 (saves the signalman a walk!) and that would enable the bridle to be much shorter to give the released by (5,6) locks 9 function which, at first glance, is all that lever needs. It also gives the signalman a nice 5,6,7 pull-sequence.
Similarly, since 4 is released by 8, it might be worth thinking about having the current 4 and 8 or alternativly the Xovers 5 and 9 concecutively numbered. (though equally it might not as it might complicate other things!) Now I am not saying that either of those things IS a good thing to do - just that when you come to the locking, don't feel constrained by what is already committed to paper!!
I don't think I am giving away any state secrets if I mention that the next MRJ (currently in preparation) has the first of a 3-part article on mechanical and electrical locking using the society MkII bits (for a 70-lever frame). In that case, we already had the prototype diagram to follow, but when I looked into the locking, it became clear why they had chosen certain particular numbering options! You will also see an example of an "always-off" signal (actually a slot) - lever 53 - in similar a situation to your 15.
If you are not a subscriber, it might be worth getting hold of a copy. I would like to be able say that the article is written by someone who knows what they are talking about, but it is actually written by me ...
Good luck with an interesting project and I hope you will post constructional progress!