Re-balance requests from a single client

do you know/can you explain why LB::reselect can't be used within HTTP_REQUEST, but LB::detach and LB::select can be?

For lack of a better answer, this has to do with event timing. The HTTP_REQUEST event triggers when the VIP consumes the request headers, but has not yet made a load balancing decision. You can preemptively cause a load balancing selection with LB::select (and perhaps detach that with LB::detach), but you can't reselect something that hasn't been selected yet.