For Q2, I suppose you see inside the file something like
tree;1
tree;2
and so on. If that’s the case, just getting the tree without numbers (Get(“tree”)) should be enough, as ROOT takes the highest number by default, giving access to all data. Read about namecycle by searching at Get Started - ROOT (use the top right loupe icon), or search for that word on this forum or documentation; e.g. Aux;1 aux;2 aux3;