Why not just

static void ReplaceDFColumn(PandasDataFrame &df, const char *col_name, idx_t idx, const py::handle &new_value) {
	df.attr("__setitem__")(col_name, new_value);
}

* This is same as used in FrameFromNumpy.

In Python, we wouldn't perform two operations to replace a column (drop then insert in position). We'd just set it: df["mycol"] = ...., which preserves position.

... As a general rule, explicit "inplace" params in Pandas is something to avoid. See https://pandas.pydata.org/pdeps/0008-inplace-methods-in-pandas.html for a longer discussion of if.