Rest assured not all of us are field engineers. Some of us continue to use mathematics outside of assignments and are interested in the theoretical background.
"Remnant flux" is indeed a misnomer for Br. It should be called remnance (or residual flux density in full). The reason we can typically avoid the confusion between B and [φ] is that for a certain transformer they are proportional. Same story for H (field strength) and I (current). Which is why you might see the B-H curve for a transformer is actually labelled flux vs current. As long as you don't change the magnetic path area (the constant of proportionality between B and [φ]) or the winding length (the constant of proportionality between H and I), the curve will look the same.
I think juleselec and PHovnanian have covered the reason why remnance is less than max B.
As for calculating an optimal angle, you ask:
Assuming I want to compensate for a known remnant flux in a controlled switching algo for a transformer is there an relationship between the BH curves and the optimal angle?
I think you'll find that the B-H curve is not relevant. It is relevant if you're trying to determine
maximum remnance, but your assumptions is that the actual remnance is "known". Note that that is a big assumption - of course, it will change depending on where in the cycle de-energisation occurred and it is very difficult to measure in practice. So let's assume it's known!
The only other thing you really need to know to determine the angle for re-energisation is the "prospective flux" that would result from application of the primary voltage. It is proportional to the integral of the voltage (I think this is the volt-seconds you refer to). In an ideal world the constant of proportionality is equal to 1/N where N is the number of turns. Non-ideal effects will likely dominate so you'd probably need to measure the maximum flux during operation at nominal volts to get the proportionality right. Note that this max flux will not be the same as on the B-H curve since in normal operation you want to keep flux below the maximum (saturation) point of the B-H curve. Let's call this max operating flux [φ]m.
Once you have your prospective flux, you'll find it is a sine wave 90 degrees lagging the line voltage (due to the integration) with an amplitude of [φ]m. The optimal angle then is simply when the prospective flux is equal to the residual flux. Note that this will occur a couple of times per cycle - either will do. If you define [θ] as the angle of the line voltage, then prospective flux [φ] = [φ]m * sin([θ] - 90[°]) = -[φ]m * cos([θ]). For [φ] equal to the residual flux [φ]r, [θ] = arccos(-[φ]r/[φ]m).
Now in reality this is very difficult to put into practice. Firstly remnance is difficult to measure and it will change every time the transformer is de-energised. Secondly, energising one phase will affect the remnance in the other phases, so the optimal angle for them changes. Thirdly, the actual moment a contactor makes the circuit is subject to unpredictable variation due to closing time and prestrike.
I found these references useful:
A. Ebner, M. Bosch, R. Cortesi, "Controlled switching of transformers - effects of closing time scatter and residual flux uncertainty," Universities Power Engineering Conference, 2008. UPEC 2008. 43rd International , vol., no., pp.1-5, 1-4 Sept. 2008
J. H. Brunke, K. J. Fröhlich, “Elimination of Transformer Inrush Currents by Controlled Switching – Part I: Theoretical Considerations,” IEEE Transactions on Power Delivery, vol. 16, no. 2, pp. 276-280, April 2001
A. Ebner, "Transient Transformer Inrush Currents due to Closing Time- and Residual Flux Measurement- Deviations if Controlled Switching is used"
