Yes, proof of 500k required for 30 days before application. These were part of guidelines sent out by Immigration HQ to local offices just prior to the first 6 months of DTV in place.
👻_I_DU You can extend once per entry. The requirements to extend are pretty much the same as you submitted to embassy initially. There might be slight variations at each Immigration Office.
Currently, you would only need to show again for an in country extension at an Immigration Office. Yes, an IO could, at their discretion, ask upon entry. It has been reported but very rare and sources are not 100%
Once more as you seem incapable of understanding English. The visa a dependent of a DTV holder is issued with is a DTV visa. They do NOT get an "O". Now get back to your bottle or you will be having the DT's not DTVs.