Robert,
I can't get the proxy to work with ClientID/Secret using credentials for 1) the internally-shared AGOL Web Map called by my local WAB app, 2) my registered WAB Dev installation (unshared), or 3) the registered, internally-shared WAB app on my local server.  If I make a proxy.config with either of those, the internal app can be accessed without entering AGOL credentials for a few minutes, after which AGOL credentials are required again.  Adding the "rateLimit" lines mentioned above does not help.  But I'm still testing...
My AGOL username/password in the proxy works fine, and is persistent.  Perhaps this is the best/only way to go for my situation.
Cheers,
Barnaby