For anyone else looking at this, this is what finally worked for me:
as a refresher, i am running Debian 9 (stretch), amd64.
I ran the following before doing anything with the ESET installer:
apt remove libssl-dev:amd64
dpkg --add-architecture i386
apt update
apt install libc6:i386 -y
The first line was a suggestion by ESET support, and it does indeed allow the installer to run. If you do that all by itself, if you are running the amd64 arch, the eraagent service will work, but the actual server security agent (esets) will not start, as it is i386...
In order to get esets working, you need to setup multiarch support, which the other three lines do.
Once everything is successful, run the ESET installer, and everything should work.