Why not try the time-honored method of changing like 1 word in each sentence (have we not all been approached or hired numerous times to do just that) and “publish” a “new” book? I’m sure you could get quite good artwork from students for a small price. I’m not saying this is particularly ethical, nor that it would necessarily avoid the ire of said buxiban chain, but it might allow you to avoid legal action (I’m assuming this due to the widespread nature of the practice in TW, which would lead me to believe that prosecution of such things is either too expensive or too bothersome to be worthwhile for the original authors).
Of course, you might want to consider how well connected the original authors/buxiban is/are…if they have some friends who don’t flinch from violence, you might want to seriously consider adopting other materials.
Just as a matter of interest, what is so great about this particular set of materials?? Is it a very well-known name, which would have marketing value with the parents, or are the materials really really effective, or what??