Yep, the pit water is full of fish. There were some ideas...

Yep, the pit water is full of fish. There were some ideas bandied about for aquaculture as a use for the 'orphan' pit once upon a time.
The country rock does have favourable qualities (i.e. calc-silicate and carbonates) which buffer out any acid generation from oxidation of sulphides (minor accessory mineralisation pyrite, arsenopyrite, Moly S2 etc).
There are flocculant treatments (eg cheap AlOOH compounds) available to 'bind-up' and 'drop out' any metals in the water, prior to any drainage.
I believe the water would be best stored for mining use, rather than 'let loose'. This would require some thought, however. Maybe III could 'prime and siphon' rather than pump it all.

As for infill DDH, III would (IMO) be mad to consider getting it from the surface. The infill would cost about half as much to acquire from the pre-mined decline (500m). For example, two underground rigs could drill the approximate 8km of infill required in about 3-4 months. This would cost approximately AU$2M. To get the same coverage (indicated to measured ~25-50m spacing) from the surface it would take about 2/3 more metres and would cost closer to AU$4M. (IMO)
It would most likely be more desirable to drain the pit prior to underground drilling, BUT, this may not be necessary. Depends on conduit through the country rock, whether the decline can be pumped dry and kept dry, for UG drilling with the pit still full. (200m+ pressure head would have to be dealt with obviously).

As for environmental concerns, III ahs an opportunity (IMO) to be innovative, show leadership and demonstrate 'good practice'.
The QLD Gov. should help out here (IMO) because the "orphan" pit was already there.

Of course, the decline will require an as yet undetermined amount of "rehab" prior to access for drilling. III has mentioned AU$3.5M for draining and infill DDH. I think these figures are pretty reasonable. I have no idea on the sort of ground support the Roche Bros. used back in the 80's, so rehab costs would be hard to estimate at this point.

Something worth keeping in mind is the down-dip potential on the "wrong" side of the so-called South wall fault. This fault appears to post date mineralisation and the movement (sinistral thrust) opens up the possibility of an undiscovered 'offset' portion down-dip to the south. Depends on the depth to Granite basement really, (IMO). This could be investigated as part of infill from the No 1 sump area of the decline. So far III have only really talked about the potential (rightly IMO) to the north and below the pit on the "right"(NE) side of the fault. Also, the decline offers an UG platform to test the northern extensions (IMO). These concepts are included in my 8km guesstimate.