sc12 = {{{1, 2, 0, 0, 0, 0, 0}}, {{3, 2, 1, 0, 0, 0, 0}}};
pnscc2 = {{{3}, {4}, {5}, {6}, {7}}, {{3, 4}, {5}, {6}, {7}}, {{3,
     4}, {5, 6}, {7}}, {{3, 4, 5}, {6}, {7}}, {{3, 4, 5}, {6,
     7}}, {{3, 4, 5, 6}, {7}}, {{3, 4, 5, 6, 8}}};
Table[Table[
   If[Length[pnscc2[[i, j]]] != 1,
    pnscc2[[i, j]] = Reverse[pnscc2[[i, j]]]], {j,
    Length[pnscc2[[i]]]}];
  pnscc2[[i]] = Flatten[pnscc2[[i]]], {i, Length[pnscc2]}];
Table[If[Depth[pnscc2[[i, j]]] == 1,
   pnscc2[[i, j]] = List[pnscc2[[i, j]]]], {i, Length[pnscc2]}, {j,
   5}];
pnscc2;
k = 1;
newlist = ConstantArray[sc12, Length[pnscc2]];
While[k < Length[pnscc2] + 1, j = 1;
 Table[If[newlist[[k]][[1, 1, i]] == 0,
   newlist[[k]] =
    ReplacePart[newlist[[k]], {{1, 1, i}} -> pnscc2[[k, j, 1]]];
   j++], {i, 7}]; k++]
newlist

{{{{1, 2, 3, 4, 5, 6, 7}}, {{3, 2, 1, 0, 0, 0, 0}}},
{{{1, 2, 4, 3, 5, 6, 7}}, {{3, 2, 1, 0, 0, 0, 0}}},
{{{1, 2, 4, 3, 6, 5, 7}}, {{3, 2, 1, 0, 0, 0, 0}}},
{{{1, 2, 5, 4, 3, 6, 7}}, {{3, 2, 1, 0, 0, 0, 0}}},
{{{1, 2, 5, 4, 3, 7, 6}}, {{3, 2, 1, 0, 0, 0, 0}}},
{{{1, 2, 6, 5, 4, 3, 7}}, {{3, 2, 1, 0, 0, 0, 0}}},
{{{1, 2, 8, 6, 5, 4, 3}}, {{3, 2, 1, 0, 0, 0, 0}}}}