Sizing/priming the reverse of the canvas

You are right that we have discussed similar issues in the past. The following thread is germane:
https://www.artcons.udel.edu/mitra/forums/question?QID=216
That does not cover all aspects of your question, though. If you read the above link you will see that many of the issues of covering the reverse of the canvas result from eventual embrittlement or not covering the whole of the canvas creating regions that would respond differently to environment changes and, therefore, would eventually create planar deformation. If one sized the from and back of a whole section of linen with an acrylic dispersion medium or a PVA dispersion size and then stretched it over a stretcher or rigid support (meaning that the whole of the image area had size on the front and back) I do not see why that would be a problem.
Certainly one should not put animal glue size on both sides of the canvas. There are problems with animal glue size under a ground but it would be a real major disaster to have it on the reverse of the canvas where it has immediate access to the environment.
However, as I wrote in that earlier thread, most of what you are trying to achieve by coating the back of the canvas could be accomplished by either stretching the canvas on a rigid support or by installing a backing board on the back of the stretcher chassis.
It is true that we suggest stretching fabric over, or adhering to, a rigid support. This does provide many benefits (see our resources section for particulars). We also realize that the added weight, expense, and other factors will mean that some will not be interested in going that route.