Any finally to the DT internals - see attached - to my eyes it doesn't match any of the colour coded inserts so 8-speed??
Most certainly 8 speed.
I think you need a newer rear mech but even they can be tricky
Now this is getting tricky.
Firstly,if your rear mech looks like the one below then it's definitely 8-speed compatible, no question about it.
Not wishing to state the obvious but I've got a feeling the issue lies within the iincompatibility between your 8-sp cassette and your 8sp DT shifters. The A/B rear mech ran from 88-92 and your SL-01RE CG Record DT shifters ran from 92-94 which tells us that they were never really designed to work together. Now, why would one 8-speed system be different to the next? Well, Campagnolo did in fact have three Syncro systems, Syncro I, Syncro II and Syncro 8sp, your DT shifters are Syncro 8sp. Syncro 8sp came out in 92 at the same time as Ergo levers but it was the first time Campagnolo made kit that wasn't backwards compatible with older potentially similar kit. I suspect the brief was to make revised DT syncro shifters that worked with the new Ergo-specific rear mechs. I know the center-to-center cog spacing required for the the post-82 DT shifters (Synro 8sp) is 5.0mm, I fear your cassette maybe different. In which case my suggestion would be to either change the shifters to Syncro I or II or update the cassette, your lovely A/B rear mech should work with either.
How does this sound?