If you look the stats from a connection with G.INP, they show that there is FEC on both bearer 0 and bearer 1.
The overhead that goes into the separate (much smaller) bearer 1 is not G.INP specific. It's the general management messages that a present with all VDSL2 connections. Perhaps it gets put into a separate bearer channel because those messages do not need to be protected by retransmission, and it'll be simpler for the other end to just re-send the message if it doesn't receive the expected reply.
Bearer 1 is not a separate channel only for retransmitted data.
Without G.INP, the management overheads are mixed in with bearer 0 (look at where the OR values are in the stats).
The net data rate is still the net data rate, you don't need to subtract any G.INP overhead from it. The net data rate for bearer 1 is zero, there's no user data, it's all management overheads. If G.INP is working hard and doing lots of retransmission, it will affect the throughput, but only while the retransmission is happening. G.INP uses its own performance monitoring values, such as leftrs (low error-free throughput seconds, recording the amount of time the throughput was below a threshold) and mineftr (recording the minimum error-free throughput).