I will try and provide some perspective from what I have read or seen in various photos and videos about the case.
1. I agree it is speculation as to how the actual abduction took place. I would want to ask the other lifeguards that worked there the question, "If someone came down the hill behind the beach at 10 am on a Tuesday morning would that seem out of place to you?" From the pictures and videos I have seen of Commins Pond Beach area it seems very secluded. Maybe Molly Bish simply did not notice someone approaching from the hill?
2. On the day of the abduction, police used scent dogs which tracked up the hill to the path in the cemetery. However, a sand truck driver who was there at Commins Pond parking lot around the same time as Molly Bish arrived for her shift said a white car had come into the Commins Pond parking lot shortly before Molly and her mother arrived. I could never understand why the white car would do this, even if the sand truck driver had not been there that day. As for the sand truck driver, I cannot figure out whether he was still there when Molly Bish was dropped off or if he had already left the Commins Pond parking lot before Molly Bish got out of the car to walk to her post or possibly to some storage shred on the property(to get lifeguard things I assume).
3. I think you need to watch some video of the actual Commins Pond area. Satellite view can be deceiving as the actual beach area does not seem like it is very large. As for where she would sit, you are generally taught as a lifeguard to sit in a place on the beach where you can visually scan the water for any emergency situation. Asking other lifeguards where she generally set up her lifeguard stuff on the beach would also help.
I think I can offer a little more perspective as to your theory about the abduction. According to 48 Hours Mystery program, there was a picture of the lawn chair she would have sat in. But when the picture was taken, it looked like there was some binder type item in the chair. This makes me wonder if she ever sat down. Also, if you look closely at the towel she draped over the chair, this lawn chair had webbing design so I would have thought the towel would have been more flush against the webbing if someone had been sitting in the lawn chair, but it is hard to tell. But maybe a parent put that binder item in the lawn chair too when they were acting as the lifeguard on the beach?
Another reason I do not think Molly Bish would have sat down yet is because she had not made her two-way radio check in call to the police department. I do not know Commins Pond beach protocols, but my guess is this radio check with the police was to ensure there was communication in case of an emergency. This would be an important part of procedure. How important was this police radio check-in at Commins Pond? I do not know, but am speculating based on what I know about lifeguarding. From everything I have read, Molly Bish never checked in with police over the two-way radio that morning.
It is difficult to come up with theories or ideas when you do not know information or have the correct information. For example, the path where the car parked in the cemetery looks very narrow so I wanted to know if police ever found any sort of tire tracks? They had to have walked that cemetery looking for any sort of evidence from a possible vehicle if they thought she was abducted into a white car. I would also think they had to have taken pictures.
I do not know what to think of Molly Bish's abduction. It could be anyone who knew she was a lifeguard at Commins Pond or saw her at Commins Pond.