After Rod Adjust

Can't find the book. Can't find it discussed anywhere. Thought maybe the IMdB crowd might have ideas about this quandary.

Initially the Power Station and the future Lava Fields sites were tied together by the setting of the 'adjustment rods'. So, when someone went afield they were in the 50 yr distant future. When they used those transfer stations, they were returned to the Power Station at 'time zero + real time passage'. Now comes where I have questions about the operation of the machines and I think it impacts the last act a lot. After Karen changed the settings of the rods to link her to the distant future, where would one of the teenage 'explorers/transferreds' end up if they entered the Lava Field machines? Would they go back to the Power Station or would they too be sent to the future?
I see a possibility that if the far distant future is the destination, that the mute survivors may learn to operate the machines, send themselves (with metal on their bodies because they didn't know the real way to operate the machines) and end up in the far future. It might even become a religious event to 'go to Heaven' by disappearing. Their destination time would be slowly changed by the soldiers back at the Power Station minutely tweaking the rods to figure out how to adjust them.
I look at it this way. Early on, why didn't the Power Station units send Transferreds to the Lava Field units of an earlier time? They seem to send the passengers to a specific amount of time forward. That amount of time remained the same as long as the rods were untouched. They didn't send people to the most future destination. Some one going backwards didn't just go 5 weeks earlier. Why not? the machines were there then. No the machines seem to be extreme operation types. You go to either 'time zero + the amount of time in the future' or the other extreme of 'far future + amount of real time passage'. One end or the other. So if someone in a middle time entered the devices, they would be sent to the far end. The only way to get back to the Power Station is to go to the far end and then come back to the 'present'. The effect would be that the future would see a steady stream of passengers arriving at their end, dead.
How did those future people lose the knowledge of the machines? I guess that's a different story.