Will ASSURA 4.12.004-615 lnx86 works with IC610 and MMSIM61. I know it is meant for ic615 but it should be backward Open-Access compatible?
This is unlikely to work (at least not correctly).
Two reasons - one is that part of Assura runs in-memory (eg DRC) and so the compiler versions are likely to be incompatible, plus there may be changes in the API. Secondly there was a change of OA version between IC611 and IC612, so the databases won't be compatible in both directions, most likely.
Overall, I'd say it was risky. Better to move your IC version to 615 too.