Finally, saying that it's "too important" to adults to legally suppress sounds like an underhanded way of admitting that it would be too difficult to try to strictly regulate this right since they have the full measure of their civil rights, whereas underagers do not.

Exactly.

The risk evaluation argument is political, not factual.

Anything has risks. Usually we leave the risk evaluation to those who are engaging in the risk.

That we outsource the risk evaluation for minors and not for adults, though, is political, not scientific.