How can I improve this digital-output / digital input (automotive)
EDIT: The changes to the question completely addressed my earlier points. Now we can talk about the new details.



A Darlington pair by definition cannot have the low saturation voltage of a single BJT, or the equivalent low on-resistance of a MOSFET. You can get optoisolators with MOSFET outputs, which might suit your application better.

The reset input of a 555 is not a TTL-compatible pin. This becomes obvious if you look at the schematic in the datasheet. Unfortunately, I've never seen an actual specification of what VIL(max) or VIH(min) are for this pin &mdash; the datahseets seem to assume that you're tying the pin directly to either Vcc or Gnd. Again, a MOSFET-based isolator would probably help here.