ad-hoc tinkering is expected to lead to disaster

This is the assumption which I question.

To discuss it, you need to address it explicitly. You might want to start from here, here and here.

I also question the other assumption of Friendly AI advocates: that all of their philosophizing and (thankfully half-hearted and ineffective) campaign to prevent the "premature" development of AGI will lead to a future containing Friendly AI, rather than no AI plus an earthbound human race dead from natural causes.

That's a wrong way to see it: the argument is simply that lack of disaster is better than a disaster (note that the scope of this category is separate from the first issue you raised, that is if it's shown that ad-hoc AGI is not disastrous, by all means go ahead and do it). Suicide is worse than pending death from "natural" causes. That's all. Whether it's likely that a better way out will be found, or even possible, is almost irrelevant to this position. But we ought to try to do it, even if it seems impossible, even if it is quite improbable.

Ad-hoc tinkering has given us the seed of essentially every other technology.

True, but if you expect a failure to kill civilization, the trial-and-error methodology must be avoided, even if it's otherwise convenient and almost indispensable, and has proven itself over the centuries.