Yes to the blackberries.
RGC is the stuff which comes in small bottles or tins, sometimes labelled as “wine enhancer”. Even when diluted it's not the same as RGJ out of cartons because this concentrate comes from wine grape varieties whereas the juices are from dessert grapes.
Sugar seems well excessive, I will attempt some sums.
Elderberries are fairly low in acid, but blackberries can be higher, halve it.
Drop the tannin.
Yes, pectic enzyme = pectolase.
Port style yeast just means it's better suited to full bodied reds, and may have a higher alcohol tolerance.
EDIT: I've just re-read the recipe and 1kg of grape concentrate would be just mental
What volume are you working to?