I suspect that codeine + seroquel is slightly safer, because seroquel doesn't synergize with opiates and doesn't cause respiratory depression. It would also probably be non-recreational and a waste of codeine*.
Both of these proposed combinations should be safe, as codeine is one of the weakest opiates, and 90mg is not a particularly strong dose. I reckon 90mg of codeine would be comparable (roughly) to 10-15mg of hydrocodone?
Taking that with 2mg of clonazepam should be safe, assuming the person taking it isn't uniquely sensitive to opiates or benzos, and would probably give someone with a low tolerance a decent buzz.
*And saying that something is a waste of codeine is really saying something!