Nice safe Matthew, had the thin pressed makers plate off one years ago but never actually saw one out in the metal.
1- 1870s
2- Double Grip Bolt Safe
3- Anyone’s guess- try eBay and filter search to completed/sold listings only. Australia is vast and values could vary massively depending on where you are
4- Confused here, why the need to fabricate/repair a locking bolt? You describe it as in fully working order with key and the locking bolts look fine in the photo.
They made some big claims in their adverts of totally drill proof Adamantine steel,, totally unpickable Duplex locks and ’only safe that can’t be opened without a key’ etc, but haven’t a clue if in reality they were any good or not!