Maintaining correct spacing around operators

I am not quite sure, if I correctly understuud your ultimate aim. I take it that you will only use the expressions for demonstration in a notebook or as a part of an image (say, graphics). If this is true, may be using Spacer may help. Let us take your example with and without the spacer:

    Row[{"a", "\[Times]", "b"}]
    Row[{"a", Spacer[2.5], "\[Times]", Spacer[2], "b"}]

it yields

the second example has somewhat larger spaces between the symbols. One can do the same also by using the BoxLanguage:

  StyleBox[RowBox[{"a", Spacer[2.5], "\[Times]", Spacer[1], "b"}], 
  SpanMaxSize -> Infinity] // DisplayForm

I hope it helps. Have fun!

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You can also use the option you mentioned, AutoSpacing. This is an option for Cell, so we must use DisplayForm to show the result in the notebook:

DisplayForm[
 Cell[BoxData@RowBox[{4, "\[Times]", SuperscriptBox[10, 5]}], AutoSpacing -> True]
]