There's no address space overlap *at all* between the IDE and other devices. IDE has two chip selects and 3 address lines. The decodes to access other memory mapped peripherals use higher order address lines that don't go to the IDE connectors.

Even if the converter was pulling on data lines when it wasn't selected, I'm not sure it should affect ethernet (though it may affect the 4321, as I remember that could have been a 5v device).

The player doesn't do DMA, no. PIO only. The SA1100 had no external DMA abilities.

Hrm.