Thanks for the additional information. I really appreciate it.
The important thing...maybe the MOST important thing to remember is that even though I'm sure you already love him...you and your family are strangers and he is in a strange place, so taking it easy with him for a few weeks, until he adjusts to you and his forever home. I know that you want to take him out and take him for walks and introduce him to people...but this can overwhelm these unsocialized little guys pretty fast. Imagine if you were dropped in the middle of Russia...you don't know anyone, you don't know the customs, and you don't speak the language...you'd be a little overwhelmed, no matter how nice everyone was. This is what Marley is dealing with.
I actually adopted a dog myself that came from a very similar situation. The first several weeks we had him was very difficult, but with time and patience he did come around.
With my little guy, the first thing I did was get him a DAP collar to help with his anxiety over the new life he had. These are collars that are impregnated with a man-made version of the dog appeasing pheromone, which is a pheromone that nursing bitches give off to their pups to help them feel calm and secure. It's something that humans can't smell, but it has an amazing effect on dogs with anxiety and other issues. Although you can find them at your vet's office..you can also find them online at places like Amazon.com and Ebay for much cheaper. Just make sure they're DAP brand, as they seem to work better than some other versions.
I would also get him a regular, hard sided kennel for in the house. I know that you're probably worried about that because we humans view kennels are punishment or somewhere to put the dog when they're not behaving...but to him, a kennel is safe...it mimics a den in the wild so it's where he's going to want to retreat there to feel safe. My little dog was HORRIBLE until I got him a kennel...I put it in an out-of-the-way spot and when he was feeling nervous or scared, he would go there, knowing that he wouldn't be bothered there. At this point, using a kennel will give Marley somewhere to go when he's feeling like this is all too much.
I'd also get him involved in a basic obedience class. Not because you want him to learn to sit and stay (although that's super nice too), but because these kinds of classes will build the bond between you and Marley and will help him build self confidence. Aussies, possibly more than any other breed, love to have a job and to please their owner, and get a great feeling of satisfaction doing one...what you're going to learn in these classes you can take home with you. Telling him to sit might be a fun thing for you, but for him, it's a job. That will help him come out of his shell a bit.
Finally...make sure you keep all meetings with new people, including your husband, on his terms for now. If he's timid, allow him to be. Ask new people to avoid looking at him, talking to him, or trying to pet him. For a frightened dog, having someone invade his personal space will be a super intimidating thing. Ask new people to just sit and ignore him.. If he comes near them, they can toss him a bit of a tasty treat (My dog loves the freeze dried beef liver you can pick up at any pet store), but not to talk to him or try to pet him. Eventually, he'll associate new people with something good (the treats!!) and will be more likely to want to socialize with them.
I hope this helps...and if you have any other questions about Marley, please don't hesitate to ask.