There is already a curve from eastbound King to northbound York. There are no curves (or track!) at the moment at York & Adelaide. I do not recall seeing any diagrams of the curves they have planned and, I assume, ordered for York & Adelaide but suspect it will not include northbound York to eastbound Adelaide. As Drum says, getting special track made takes months. There is no southbound track on York yet but they have been planning to add it from Queen to Adelaide (and ought to bring it down the last short block to King for system resilience.)
The curve is westbound King to northbound York only, none for eastbound, unfortunately. Also I'm not suggesting a detour that requires northbound York to eastbound Adelaide.
What I am suggesting is only the eastbound 501 detour be changed from Queen-York-Adelaide-Church-Queen to instead be Queen-York-King-Church-Queen. At least for most of those 20 months. The westbound detour can stay on Richmond.
Yes it would likely require a new track order, though if you could act quickly, this may be a far better option in the short and long run.