There are a few reasons why the owner isn't budging (some as stated aboved):

1. Vendor not motivated
2. Vendor or Ex are being difficult 
3. Price has not been adjusted in line with market correction
4. Agent has overquoted and being held to account
5. Bank is owed more than your offer and owners are not able to accept less

Stick to your guns – if necessary tell the agent what the low-medium range of the valuation was (and use this as the max that the bank will approve), you have got the evidence – use it (you have paid for it so you have nothing to lose if they still want something above your walk away price).

If you are looking at going owner-builder, there is a short course which covers the requirements (at least in NSW).

Jay, if the agent has received an offer then there is nothing stopping you from submitting one also. The agent must present all offers to the vendors (make yours in writing with a sunset clause).

In the unlikely event that the agent is lying, then they have your offer& conditions to present to the principal. They can always come back and ask you to increase the offer.

An agent is required to present all offers to the owners, they don't however need to tell prospective buyers whether those offers are still under consideration by the owner ie if they have been rejected. So telling you that the owner has received offers is true (but not disclosing that they have been rejected is also OK).

Brisbane/GC are both markets which go gangbusters for a few years, plateau, drop then  plateau again.

Not quite the answer you're after but something to consider: do you have enough in super left to cover your debts? Trigger a release event by working overseas (with the intention never to return) & pay out debts. Work OS for a year or two, then start fresh.

A personal loan is bank loan with a rate higher than your IO loan but lower than your credit card. The rate is higher because it is unsecured but if you have equity in the property, it may well be much more effective to take out a line of credit against that property. Have a chat to your bank or broker.

If you want a safe projection, look at longer term growth for the area ie longer than the 7 or 10 years as this barely takes in one cycle ie. you may be taking into account two growth periods or two relatively stable periods in your timeframe.

Too hard to comment without knowing what state you’re in.

Basically a subdivision is the creation of a new block ie you would have 2 blocks with separate titles, dual occupancy is 2 dwellings on the one title (either attached, detached or Granny flats). With DO you will still want to consider separation of services so that the tenants are individually metered and will pay water/power etc.