Secondly the discoprise was shown to be 288 meters in an in-universe graphic
An outdated graphic I'm afraid. All the info is pulled from the old Joseph technical manual I believe (basically copy-pasted without care I assume), and the orthos of the Enterprise is that of the John Eaves design before the CG guys tweaked it (most noticeable difference is the orthos has straight pylons still).
Besides, based on what we see on screen there is no way the TOS Enterprise is only 288m long.