There's no contradiction here. As CR706.2 lays out, the text-changing effect applied to the original spell by its Overload ability isn't copied. What is copied, however, is the Overload ability itself, and it's the copy's Overload ability that then applies its own text-changing effect to the copy.
This example may be easier to understand using permanents. Let's say I control
Aeronaut Tinkerer and an artifact, so my Aeronaut has flying. If my opponent (who controls no artifacts)
Clones my Aeronaut, their copy will not naturally have flying, because flying is neither written on the card nor applied as part of another copy effect, so it's not part of the Aeronaut's copiable characteristics. If I
Clone my own Aeronaut, though, that copy
will have flying, because even though flying itself isn't copied–just as it wasn't for my opponent–the Aeronaut's “this has flying as long as you control an artifact” ability
is copied, and that ability then gives my copy flying.
The same thing's going on here–the changes made to the original spell by the original's Overload ability aren't copied directly, but the copy gets its own instance of Overload, and that instance of Overload then makes changes to the copy.
Copies of
Firespout (and similar) don't do anything because the copy checks on resolution what mana was spent to cast it, and sees that no mana was spent to cast it at all. When 706.2 talks about choices made when casting the spell, it refers only to the choices laid out in CR601.2b-d–modes, additional and/or alternative costs, value of X, targets, division among targets, and so on. The amount and type of mana that you ended up spending to cast the original spell are not among those choices.