It's very unlikely that the Ethernet connection will negotiate at 1000BASE-T if you simply interconnect the LAN ports of two collocated AC+DC Enhanced Power Injectors. 1000BASE-T does not tolerate very much loading on the cable, and an Ethernet connection with a total of four LPUs and two AC+DC Enhanced Power Injectors does not normally negotiate at 1000BASE-T, even when the combined cable length is less than 100 m.
As you have observed, 100BASE-T is more forgiving.
The problem is normally resolved by installing an Ethernet switch at the "B" point in your network. The switch regenerates the Ethernet signal.
The fact that you got 1000BASE-T with the laptop suggests that the wiring is all good.