This is obviously unmeasurable - there isn't convincing analysis because there can't be convincing analysis, as you admit. The fact that people were willing to

This is obviously unmeasurable - there isn't convincing analysis because there can't be convincing analysis, as you admit. The fact that people were willing to spend an amount of money that breached electoral law in the UK, and presumably even more in the US, suggests that there was some reason for them to do so. This happened only because experts in this field believed it would influence the outcome of the election. That's your evidence.