With the tall glass viewing windows already there, seems to me the purpose of a moat is rather moot. Filling it in would double the usable space of a really inappropriately small exhibit IMO
Also if you have any photos of the elephant exhibit and recent expansion id love to see them
I'm not sure the glass would be high enough without additional panneling or hotwiring, especially for primates that climb or jump better than mandrills and with climbing structures closer to the window. Without higher and/or larger climbing structures I don't think you could effectively get primates an improved space.
The current mandrill group is on the elderly side and dwindling, they appear to phasing this species out. Not sure what they will do with this exhibit in the future. But this whole building needs a major overhaul or honestly a complete demolition, redesign and rebuild, hopefully not too distant in the future, although I don't see them doing anything about it before they are done rebuilding the jubilee complex (if that ever happens).
With regards to the elephants, here are a few of the expansion and the original paddock I posted previously. If I have any more recent ones I will consider including them in future uploads.