I'd like to offer a theory of how the girls could have been taken against their will.... in broad daylight....by a stranger....without anyone hearing or seeing anything. The girls cut through the woods taking the shortcut back to Jennings Rd. The nursing home parking lot would have been to their left...on the other side of the trees. The perp could have backed into a parking stall...popped the trunk but not opened it all the way. He might not have known the girls but he could have known the area and the habits of the neighborhood kids....such as the shortcut. He waits for a child to use the shortcut...steps out from the trees with a gun...tells the girls to come quietly. It would only be a few steps through the trees to the trunk of his car. That could be done in under a minute and he wouldn't have to try to control them or have them be seen while he's driving because he used the trunk. This can be done and has been done. Sisters older than Sheila and Kate went quietly.....
Your theory has some good elements to it, and might explain how the girls disappeared.
It would have to be based on the perpetrator knowing exactly where the path went and that the girls would be using it on their walk home, and knowing their time schedule fairly accurately.
There would be an elevated risk factor in him using the Nursing Home grounds as his ambush point. It would be obvious to him that someone might see him from one of the many windows looking out on the parking lot. More chance of someone sitting and looking out a window here than from one of the houses along the girls' probable track. And the longer he had to wait, the more chance of being seen.
A vigilent grounds keeper was mentioned by someone in a post. There would have been the chance of him seeing someone hanging out at the parking lot.
Since the pond on the property was later searched by police divers, the grounds keeper would definitely have been in contact with the police. He may have been questioned by police and certainly would have had the opportunity to mention anything that he saw which he thought was suspicious.
Your scenario is certainly possible. An abductor could have used a gun or simply some ruse to get them into his vehicle.
I think it is very possible that the girls were abducted close to the Nursing Home. I would suggest, however, that a better place for him to have waited would have been on the other side of the Nursing Home pond - closer to the Drumm and McComas intersection.
This would have offered three advantages over the Nursing Home parking lot:
First, If he were parked on Drumm, facing across McComas toward Devin, he would be able to observe the the girls approaching him on Drumm.
Second, he would be somewhat less visible to possible witnesses. If, as the girls reached him, he was to see another car approaching or someone watching, he could call it off, and simply drive away.
Third, he could cover four possible routes that the girls might take when arriving at the intersection: left on McComas, right on McComas, straight on Drumm, or half left to enter the foot path. This way, he would not have to know exactly what their intended route was.