I wound up getting past it with something like:

val assembler = new VectorAssembler() .setInputCols(Array("f1", "f2")) .setOutputCol("features")

val labeled = assembler.transform(df) .select($"price".getItem(1).cast("double"), $"features") .map{case Row(price: Double, features: Vector) => LabeledPoint(price, features)}

which is seems much less verbose (h/t stackoverflow) and directly "promotes" the struct's elements to where I need them.

I also wound up getting past the exceptions (the were, IIRC, match exceptions).

Thanks for the lean-in.