It all depends on how it's implemented but yes, it is certainly feasible. Let's say you have network segment A containing systems which handle cardholder data, network segment B with AD servers and network segment C with systems which do not handle cardholder data in any capacity, do not have access to segment A and management of systems within A and B is performed using a bastion host or jump server within segment B. In this scenario, there could be effective segmentation and, depending on security controls implemented, you should be looking good for using the same AD throughout. Throw in proper Role Based Access Control, monitoring and logging, risk assessment etc and you're getting closer to compliance.
As for the proxy, is the proxy transmitting cardholder data and what security posture/risk has it? The likelihood is that the proxy server will be considered to be in-scope though it could be used by both in-scope and not-in-scope systems.