AFAIK after 1) Jitsi should send subscribe message to xcap defined list. After this kamailio can send individual subscribe packets in backend.
But as you described - Jitsi does not send this magic subscribe. Problem application is Jitsi - not kamailio.
I spend couple of weeks diving into xcap/rls stuff and had the same problem with the jitsi.
Finally i end up with own softphone code (based on resiprocate) - it works ok. I'd suggest you to try with bria or blink softphones - maybe they will perform better.
If anyone got rls working with jitsi - please share your experience...