Using EJEJC_multiAdd and PDF Stamping both work just fine together when they are implemented in the standard manner we have documented, so other sellers who need to do that typically don't have any problems with it.
The problems you encountered were happening because you were trying to implement EJEJC_multiAdd on your page in a nonstandard fashion, which we did not recommend and, frankly, could not readily figure out what exactly you were trying to do, or how you were trying to do it, or even why you were needing do it that way instead of our standard way. There was no problem in our software, only problems in the "homebrew" technique you were developing on your own to use our software in an unfamiliar manner that we'd never intended for it to begin with.
To make an analogy, if you try to install a Ford engine in your Chevy, and you cobble up a bunch of custom handmade parts in your garage to make it fit, your local Ford and Chevy dealerships probably aren't going to be of much help in sorting out any problems you may have with those custom bits, as those problems are unlikely to be inherent flaws in the Ford engine or the Chevy body themselves; that's why you'd do better to seek solutions for those problems at a custom hotrod shop, such as Andrew's business. :^)