I'm not absolutely sure, but I think it depends on your situation. Stitching of user session to a purepath is done via Dynatrace cookies (dtCookie and others).
If you have PurePaths, then you have OneAgent installed on the server side. Why are you not using the OneAgent RUM injection and you are trying to do Agentless RUM instead?
Ok, so basically if you have a static application, it should be enough if your services are monitored by oneagent and RUM is not disabled for the process groups. As soon as a request from a real browser is sent to a web service/web request service monitored by oneagent, the cookies will be set, unless you have disabled the RUM functionality for the process group or at the application level in Dynatrace.
In your case, you won't have any PurePaths of the files hosted on S3 as there is no agent to capture the purepaths. Where are your services hosted?