Wago 750-880 Modbus Coils - How to adress correct

That’s rubbish. You’ll be here a long time if you just guess stuff.

Yes, exactly right.

Don’t. Put them together in data thing.

Bridge poller slave0-coils [ ... details for read polling from 512  ] {
   Thing data slave0-do0 [ [ readStart="512",  ..... writeStart="0" ... ]
}

EDIT - found an old post using Wago with Modbus-1 binding, suggests you should read from coil 512 and write to coil 0 , then 513 - 1 etc
Inputs and Outputs via Modbus with Wago 750-352 / Openhab 2 - #3 by mhsob.