How can I locate the web.config for a host-named site collection ? or Which web.config can I use for making web.config related changes for a host-named site collection ?
All (related) host named site collections are sharing the same web.config as they are all located under a single web application (= entry within IIS).
Look into the web application list (central administration) to find the parent web application and then adapt the relevant web.config within IIS.
Please be aware that I'll have impact on all other HNSC.