If the effective concentration is 5 uM and it is cytotoxic at that level, then it is highly unlikely you can use it for anything drug wise - it will kill the cells. This is fine if you want to kill cells (including "normal" cells), but it isn't if you want to do anything else. The Ec50 should not vary based on assay - it should give you the same value (+/- a bit, there is variability but it shouldn't be a log10 out). To do an effective EC50 you should have 10-15 data points from difference concentrations of drug, some above your concentration and some below, then have plotted the sigmoidal curve. If you haven't done this, then moving on to other assays is a bit pointless.
Like I said above, inhibition of proliferation and cell death are two different things - if you can't measure loss of proliferation without encountering cell death then there are no assays that will work.