No american carrier offers free upgrades to J, you have to use a certificate (unless it's an oversold and you're lucky enough to get an Op up).
If there were seats available why didn't you apply a GUC? That's a long flight and a good use of a GUC.
I agree that the RUC's and GUC's are hard to use and wish Delta would allow them to be used as long as the cabin is less than half sold.
Even if there were seats showing a few days out, it's possible there were some people on GUC's waitlisted that hadn't cleared yet. I'd rather Delta give those seats to diamonds using a GUC than to sell them really cheap.
Agree totally about lounge access. As profitable of a route as that is for DL they should provide partner lounge access to diamonds in Y.