Based on your story, I think your lawyer should take lots of responsibility of voiding the contract at your end. Maybe you should sue your lawyer Stephen Guest for compensation. Because of him, you can not finish signing the full contract on the closing day, which later becomes the primary reason that Minto refuses returning your deposit. You lawyer should be well aware of the consequence he caused intentionally or not. Did he warn you about what will happen if you don't sign the full contract on the closing day? For him or his assistant, leaving early is not an excuse and he should be responsible for it. If he didn't tell you the consequence, he should be under investigation for his legal malpractice.
I think everyone should be cautious when signing contract with the builder, always keep photo copies of signed contract. When feeling something not right, back off quickly.
As for Minto, I hope they will give back your full deposit eventually. They can make mistake of the house model, and they tried to correct or made an alternative offer, but it was in your hand whether you wanted to accept. However if they don't sell the house to you, they should NOT keep your hard earned money. If not, they will lose many of their potential clients and business.
Please take care yourself and your health is precious.
Learning to move on and not let other people's fault ruin your life.
Keep fighting smartly and strategically!
Good luck on getting back your full deposit!