Not so! The only limitation on the Carousel IV INS when first installed in the VC10K was the lack of a TAS feed in the early 'breadboard' installations done on-squadron. But that was rectified during the 'official' mod. programme. The 101 Sqn JEngO was generously awarded the 'Cock Up of the Month' trophy for having cut several lengths of expensive cable needed for the 'breadboard' mod., only to discover that the tape measure he was using had 18" missing from the end, so all the cables were useless....
The FMS 800 RDU, which is the display to which you refer, most certainly
did display routes included in the navigator's CDU - in white, not magenta. But that appeared about 15 years after the Carousel; at the same time it replaced the Carousel IV repeater in the VC10C1K.
I have only seen an 'HSI' presentation once on the VC10K FMS RDU - due to faulty software, bizarrely it appeared and confused the heck out of us. A shut down and reboot produced the normal display a few minutes later though.
I agree that that we should have had the 'full' FMS 800 though - and with a second RDU for the navigator.