What is the ERC (European Research Council)? Find more information here:

-ERC website
-Euresearch: the Swiss guide to European research & innovation

Q: Would I be eligible to apply for an AdG 2018, whereas I am involved as coordinator in a Synergy proposal submitted in November 2017 (SyG-2018). What happens should that grant get funded?

Indeed, if you submitted to the SyG-2018 as PI, and if your proposal is eligible, you are not eligible for the ERC AdG 2018, as this call is under the same WP. The AdG proposal would be declared as ineligible and not be evaluated.