I've seen claims that apparently the oldest Sarasvati of Mandala 6 is actually from Afghanistan, how accurate is this claim?
There is no way to prove or disprove such claims. You have mentioned the Sarasvati hymn in Rig Veda VI.61. There is another mention of Sarasvati in Book 7.
This stream Sarasvati with fostering current comes forth, our sure defence, our fort of iron, as on a car, the flood flows on, surpassing in majesty and might all other waters;
Rig Veda VII.95.1
Pure in her course from mountain to the ocean, alone of her streams Sarasvati has listened. Thinking of wealth and the great world of creatures, she poured for Nahusa her milk and fatness.
Rig Veda VII.95.2
Sarasvati cannot possibly be an Afghan river if she flows to the ocean. However, scholars like Witzel and historian like Irfan Habib have claimed that ocean means here a big lake.
The answer to the question is it all depends on how you interpret the word 'ocean'.