Back to placing the cakes directly on separator plates, this can be done and cutting the cake doesn't mark them up either, you have to cut awfully hard to cut them. I do it both ways, depending on the cake and where it is going and how it will be used. The one drawback is that because of the feet, they take up space so when figuring on the depth of your cake you also have to worry about the legs when calculating the depth of the box. The advantage to using the extra boards is that the leftover cake or saved tiers don't interfere with you getting these separator plates back promptly. The advantage to just using the separator plates is that they are much more stable than cardboard so you don't have to worry about doubling up the boards so that they don't cause any cracking of the icing when the boarded cake is being moved around. But actually the separator plates were originally designed for direct use.
I have never seen the low string glue either, low temperature and dual temperature yes, but low string never. I will have to shop for that, it sounds like a good idea and maybe I can put my hairblower away and stop using it to get rid of the glue threads!
Hugs Squirrelly Cakes