Efficient method for Inserting arrays into arrays

Q1

Join[a[[;; , ;; 1]], b, a[[;; , 2 ;;]], 2]

One may want to create function

insCol[a_, b_, n_] := Join[a[[;; , ;; n - 1]], b, a[[;; , n ;;]], 2]

insRow[a_, b_, n_] := Join[a[[;; n - 1]], b, a[[n ;;]]]

After a few attempts, the following gives a noticeable if unremarkable improvement over insertarray:

columnInsert[a_, b_, pos_] := 
  MapThread[Join[#1[[;; pos - 1]], #2, #1[[pos ;;]]] &, {a, b}];

Your second functionality doensn't actually need anything fancy, just some swaps afterwards:

SetAttributes[columnInsert`swap, HoldFirst];
columnInsert`swap[mat_, first_, list_] :=
 Module[{i = first}, 
  Scan[columnInsert`tmp = mat[[All, i]]; 
     mat[[All, i++]] = mat[[All, #]]; 
     mat[[All, #]] = columnInsert`tmp; &, list]; mat]
columnInsert[a_, b_, pos_List] := 
 Module[{res = columnInsert[a, b, First@pos]},
  columnInsert`swap[res, First@pos, pos]; res]