This link, from the post above, has 2 photos of the crime scene
http://www.rgj.com/story/news/crime/2016/03/27/unsolved-10-years-gone-musalo-murder-remains-mystery/82278524/
One shows the dirt/mud footprints leading from the front entrance of the home through the front hallway and into another room
The other photo shows the hallway outside the Musalo's bedroom. At the end of the hallway is a cabinet of some sort with the door ajar. Was the killer looking for something to steal or simply making it appear a robbery had taken place?
Photos of the lovely home show it was, like others in the neighborhood, set apart some distance from next door neighbors and surrounded by trees. Even with outdoor lighting, it would have been possible for the killer to approach the home, cut the phone lines and enter by the front door without easily being noticed.
According to the articles linked above and information provided by TomGreen1164 , we know:
The community is gated, staffed by guards who check every car coming in. Is it staffed 24-7?
The killer entered the home through the front door, at night, while the Musalo's were asleep, and killed them in their bed
The killer then exited the home via the back door, walked across their back yard and climbed a fence to the road that runs behind the development.
There were no signs of forced entry, they had intimate knowledge of the layout of the house
The Musalo's didn't employ anyone who would have had knowledge of the inside layout of the house
There was nothing of significant value taken
Family and everyone who worked at Montreux was investigated
Their last known contact with anyone was a furniture salesman at noon that day
Their computer activity at home showed they were active until that night, showing they had been at home most of the day
The road that runs outside the housing development is rather remote in this area. Late at night, it doesn't seem there would have been a lot of traffic. Did the killer have someone waiting to pick him up, or did he park his car somewhere nearby, knowing he wouldn't be able to get back out through the security gate? IMO, it sounds like more than one person involved in the plan - the killer and someone who drove him into the development earlier in the evening, then picked him up outside later on.
I keep coming back to the idea of a worker at the housing development or golf course/country club as the driver and an acquaintance as the killer. The killer could have hidden in the worker's car to get in, hidden in a shed or storage area, etc. on the premises, etc. Someone who worked at the development would likely have knowledge of the floor plans of various homes, including the Musalo's. Perhaps it was a failed robbery, with the perps somehow having the mistaken impression the couple had valuables in their home. Sort of like the Clutter family murders. The couple were killed to avoid having witnesses.
If it were someone who lived in the development, why would they leave over the back fence to walk along the deserted highway late at night.
Also, IMO, phone lines were cut to keep the couple from calling for help. Ten years ago, not everyone, and certainly not older people, had cell phones. Most were still using land lines.
Might help to look, not just at workers at Montrose, but any friends or relations they might have that were known to come to work with them or hang around.
They brought their own weapon, so the attack was likely planned in advance