And, finally, here is the solution to the missing readout in SCOPE mode and the acquisition warning which were tied to the same cause. I am documenting some insights for posterity and for the benefit of anyone who may want to troubleshoot a readout issue with the 7854 at some point.
In sch 34, the RO-WFMX/Y signals were present in all modes. However, while the WFMZON signal was present, allowing stored waveforms to be displayed, the CHRZON signal was high, inhibiting the RO display n SCOPE mode. The latter is from sch 33, and available off TP2400-12. This implied that the characters that were attempted to be displayed were invalid and so there was no dot pattern to be generated by the dot timer stage. Since the readout was OK in the STORE mode the source of the bad characters had to be in the readout acquisition stage, sch 31.
In sch 31, TP2400-26 and 28 indicated that the plugins were sending row and column pulses so U2520 and U2530 were probably OK. To check the A/D converters U1220 and U1520 (the row and columnn decoders) I tapped Row 3 and Column 10 of the outputs of these ICs (the IDENTIFY signal). Pressing the Identify button on the plugin caused a reaction on Column 10 but no reaction on Row 3. That pointed to U1220. I found that both these A/D converters were socketed indicating a prior repair attempt. Sure enough, interchanging them reversed the problem and also led to a much-desired readout appearing on the screen though it was not meaningful because of a bad column decoder.
Ordered a new A/D converter off ePray - these things are hard to find and I had to pay a good $18 for one. They do look prone to failure going by the earlier repair attempts on these same chips in my 7854.
Best - Ram