As I said above, the cumulative shift figure comes from fractional sample adjustments that were made to align with the timing reference, which would be the peak of the loopback IR for your measurements. It has no relevance for what you want to do so please ignore it. The system delay for loopback measurements shows the relative times of the loopback and measurement peaks. That may not be the best choice, I could change it to operate in the same way as the acoustic timing reference, which uses the estimated delay of the measured IR if its peak is close to the IR start, or the estimated IR start if the peak is later (typically the case for measurements of low bandwidth systems or drivers). You can do this manually by using the Estimate IR delay button, and then choosing the option to either shift the IR or to both shift the IR and use the delay estimate as the timing offset for subsequent measurements. For the measurement you posted above the estimated delay is 1.4954 ms, 513 mm. Here is how the IR looks when shifted by that amount, shown overlaid on the minimum phase IR since REW uses cross-correlation with the min phase IR to estimate the delay.
View attachment 44410
Again, if you think that figure is wrong, enter whatever figure you want to use instead, as an offset for t=0 and/or in the timing offset field on the Measure dialog for subsequent measurements.