Just a suggestion: (I assume the cove is painted and not stained finished.)
I have bent 90s on 500 WM with a 1/2" EMT bender.
It's the front pipe support (toe) of the bender that tends to distort the sweep of the 90 as it's being bent. Take care aligning the WM in the bender before you start bending the WM.
You say you will cut a slot out of the ceiling cove for the 500 WM.
I would suggest you first take a piece of blank paper or a piece of cardboard and draw an end view of the cove on the paper to actual dimensions. (Best you can)
Bend a 90 on a piece of discarded 500 WM. Place it on the drawing of the end view of the cove. Check how the radius of the bend looks. Does it somewhat fit the inside curve of the front edge of the cove?
Will it look respectable?
If it does cut the slot out of the cove for the WM. Install the WM and complete the wiring installation.
You can patch the cove using USG Lightweight Setting-Type Joint Compounds.
https://www.youtube.com/watch?v=WoyYculBKdQ
The 20 minute sets up pretty fast, (don't mix up too much at a time), so you can sand and shape/sculpture the area you are filling usually within 30 minutes. It may take a few applications and sanding to get the desire look.
Finish off by priming and finish painting.