Yes, it's possible to calculate the tension difference, in fact there is one spoke length calculator that I know of that also calculates the tension difference for you.
Haven't seen that one before. Do you have a link?
I don't think that varying the number of spoke crossings will modify the tension difference, the bracing angle remains the same.
Maybe it only works on high-low hubs with different flange diameters on DS and NDS. These hubs were designed for different numbers of spoke crossings on DS and NDS to try and even out spoke tension.
Try it on the spoke length calculator you mentioned.
1. Set the same number of spoke crossings for DS and NDS
2. Set different numbers of spoke crossings for DS and NDS.
3. Compare tensions for 1 and 2.
In practice I have a good idea of what kind of tension I want for my DS which will give me an acceptable tension for the NDS. With greater dishing it's now necessary to increase the tension on the DS to have an acceptable tension on the NDS. I can only see more wheel problems in the future. If you don't have enough tension you have a weak wheel but if you have to increase the tension, because of the greater dishing with 11 speeds then some rims will be cracking. I heard the other day that Open Pro rims are suffering from a higher rate of failure due to cracks, I think it may be the 11 speeds hubs that need more tension on the DS that are causing this unusually high rate.
Eleven speed is great for TdF, who really do need close ratios to optimise their cadence/speed relationship. But they also have pro mechanics and spare wheels following them everywhere. For the rest of us, I'd say 7-8 speed was the optimum combination of gearing choice and keeping the dishing to a reasonable level. Of course manufacturers and bike shops love eleven speed, because they charge a premium for a product that doesn't last as long.