Loco hauled via Geelong, meet with V/locity at Ballarat 6 x week return would seem the most logicial and kills a few birds at the same time as it returns service between Geelong and Ballarat as pushed by others and allows a 2nd set to be conveyed in one movement for a service on the interstate and/or Portland.
Geelong to Maryborough via Ballarat is broad gauge right now. Murray Basin Project originally had scope to convert it to SG or DG but with that whole program being a complete basket case, right now it is unclear if and when this will be completed.
Broad Gauge trains currently run direct to Maryborough via Bacchus Marsh & Creswick, connecting at Maryborough to a SG train to and from Mildura is achievable now.
If the SG train from Maryborough to Mildura proves successful, that may provide more of a mandate for the government to convert the line from Maryborough to Geelong to SG or DG to give Mildura - Melbourne pass a single train run.
While I believe it is achievable, I don't believe the political interest exists in either of the major parties. If either party provided this, all it would likely do is shore up the seat for the incumbent independent.
From a political point of view there is little to gain here.
For this reason, I believe the only option that could have a chance of success is one that requires the least possible outlay to implement.