Surprising but great that they were able to diagnose the issue properly. I agree that you should go with something tonally similar for the center channel speaker. You might consider an articulating speaker mount (similar to articulating flat panel TV mounts) that will allow you to bring the new center channel speaker a bit forward and down when in use, and tuck it back in front of the TV when not in use. This way it won't block the screen when using it but it will have some room to breathe.
BTW, are you 100% sure the C1 won't have the same problem? You could also consider electrostatic, ribbon or MartinLogan's "folded motion" drivers in their Motion series (similar to a Heil Air Motion Transformer). These produce a similarly open and transparent sound to the Mirage speakers. The ML Motion C speakers is 90 dB efficient and rated to handle up to 200 WPC. Sells for $499 and it about 6" x 24" X 4.4".
If you do buy the Mirage please use our link as we are a Vann's affiliate, and this is what keeps the site up and running:
And the MartinLogan Motion C: