I think I've taken more than enough of your time on this slippery issue. I'm pleased enough to have found a way to moderate its effect. I did have a go in Cakewalk, simply adding Addictive Keys to a project and removing it, and presto an average 25M leak on each iteration, mostly consistent across 23 times adding and deleting. I'm sorry, I should have tried that test much sooner. I've taken this to XLN support to seek their advice/explanation/assistance. If I learn anything of value I'll pass it on.
I did the same test for SI-Drum kits in Cakewalk and it did the same, though losing less memory (consistent with my own app). That surprised me since the SI VSTi's are so closely connected with Cakewalk.
Anyway, consider this a dead question for now, even though it remains unresolved for me.