2. Availability: for how many elections would this be possible?
(a) only one–Presidential–this would keep the process generally manageable, but seems overlimiting as there are other races that matter–from another perspective, even this might not be manageable if there are multiple ads airing in various states and regionally specific.
(b) very few–Presidential and maybe Senate races–this would cut down on the required resources it would take to perform this service, but stands to again decrease (overlimit) the available use scenarios.
(c) any or all on the federal level and gubernatorial elections (as these are all of the elections that most people think matter on a regular basis)–if the service gets used, it stands to be too large of a burden for any organization, and if it doesn’t get used, it doesn’t get used.

3. Campaign efficacy and commitment–is it in a candidate’s interest enough to get them to sign on to it?
(a) yes: (i) argumentative high ground goes to the candidate that uses the service–this requires that this activity be sanctioned by a particular, mutually agreed upon organization so that it doesn’t become a delegitimized, partisan poop-shoot; (ii) voters would prefer something be done to decrease the prevalence of smear commercials, so politicians would like to sign on; (iii) this would improve political discourse, and politicians may prefer this.
(b) no (politicians think strategically, and will only do something like this if it will help them win, and political parties will only endorse this if they think it will help them win most of the time, or if voters demand it, which it is difficult to identify any recent example of a majority of voters wanting and getting anything that at least some politicians didn’t want to give them): (i) commitment is key–if politicians can use this for only some commercials and not others, then it confuses viewers/voters and delegitimizes the system; (ii) but if politicians must commit long term (sign a contract to use the service for all commercials in an election cycle), then they will not be able to respond quickly enough to circumstances and immediately or pre-emptively release commercials in their interests–in some cases, this expense will decrease their chance of signing on in the first place; (iii) the skepticism of fact-checking and misleading-ness may be enough for politicians to avoid this topic–in other words, there are many issues that can be deemed open to human interpretation and framing rather than fact in popular discourse; (iv) is it possible that enough politicians will see truthtelling rather than misleading as a better course of action than saying what you can say to win?

just a few thoughts expanding on the last paragraph of the post.
–brett