Oh I see, so it's not just an issue with the hack for locating the ScrollViewer not working, it actually just isn't there. Thanks for confirming, that changes things a bit.
That does suggest that Mono's implementation is busted in some way, yeah - I wonder if I'm just using it incorrectly, but as far as I'm aware the actual usage of the FlowDocument system for presenting the game text is fairly run-of-the-mill - and even so, my usage of it shouldn't impact its internals magically not existing sometimes...
Thank you for all the help looking into this. I think that for the time being, people who wish to play on Linux now have a viable route to pursue, thanks to your efforts in writing an install script - it should do the trick for most use cases.