Well, persistence and ingenuity sometimes pays off.
I have managed to source copies of some documents that may help.
Lannanta - it looks like my summary of the Charlotte Quick marriage situation was pretty close but the jury remains out on the fate of Nicholas Pearce I think.
AH! - on a second reading of the first document I think it is your own theory that might be correct! Both Nicholas Pierce and Charlotte Quick were just a little less than truthful about their ages and the mother of the groom looks like Mary Ever. Nicholas was therefore the son of Thomas Pearce and Mary Ann Eva as you surmised earlier.
7th April 1856 At the Mission House Geelong
Nicholas Pierce bachelor born Cornwall England age 21 miner
Charlotte Quick spinster born Cornwall England age 18 housemaid
Both residents of Ballarat
Groom's parents Thomas Pierce and Mary Ever
Bride's parents Nathaniel Quick and Susan Sandow
Witnesses - Samuel Semmons, Margery Semmons
9th April 1870 Urquhart Street Ballarat West
John Stevens Hollow bachelor born St Ives Cornwall miner age 27
Charlotte Sandow Quick spinster born St Ives Cornwall age 28
both of Ballarat but groom usually resident at Avoca
Groom's parents Edward Hollow and Honor Stevens
Bride's parents Nathaniel Quick and Susan Sandow
Witnesses - John Mann and Margery Mann
These documents prove that the same Charlotte Quick was involved in both marriages and as the Pioneers Index shows a later marriage for Nicholas Pearce it also proves that Charlotte was not widowed. What we still don't know is whether the first marriage ended with a Divorce or with Annulment. The fact the marriage took place at Geelong when both bride and groom were resident at Ballarat suggests maybe an Annulment.
The final document is information from the death certificate of William Quick who died at Wiseman's Creek, NSW in 1885. It does not provide names of parents or any other relatives but coupled with the information from immigration records and passenger lists this does support the likelihood that William was the son of William Quick and Margaret Polmear.
William Quick age 72 died 26th October 1885 at Wiseman's Creek.
Cause of death - debility accelerated by starvation and neglect
Burial - 28th October 1885 at O'Connell (a small village south of Bathurst)
Birthplace shown as Penzance but time in the Australian Colonies was not known
Not all questions have been answered but most of the mystery has been solved I think.
CT