Is cell references based upon data:[…] or displayed cells? i.e. should cell references go out of range when expanding and collapsing in treeTable?
Yep, there’s omission in our docs. Indexes are refreshing on every expanding/collapsing/filtering, so in treeTable the only possible way to call some row is to use its (i.e. data) ID.
As simple as
from “R_Year”:"=[:1,:2]+[:7,:2]+[:10,:2]",
to “R_Year”:"=[1.1,R_Year]+[1.2,R_Year]+[1.3,R_Year]",
?
Tx. 
Yep, absolutely right.
BTW, in your case "R_Year":"=[1.1,:2]+[1.2,:2]+[1.3,:2]" will be correct too.