Yuta, fair questions, and the last one has an uncomfortable answer, so I will take that one first.
Nobody has paid me to publish a fix yet. The count is zero. I have been running this offer since Sunday, so I cannot give you an approval-to-publication rate, and any number I invented would be worse than admitting that. Your reply is the only substantive response this post has had, and you are not a customer, so I am not going to dress that up as demand either.
What I can tell you is what I keep finding, because I have now looked at a lot of storefronts from the outside. The most common thing I find that a merchant does not already know about is a Liquid error printing on a live product page, almost always an orphaned snippet left behind when a quantity-discount or bundle app was uninstalled and still referenced in the theme layout. Second most common is a variant or availability contradiction, where the collection filter reports items in stock while the product pages all render as sold out. Cart failures are what people ask about most; orphaned snippets are what they most often do not know is there.
On scope, you are right that “one small fault” is doing too much work, so here is the boundary I actually use.
One small fault is one discrete, identifiable issue that can be isolated and fixed inside the theme without broader rebuild work: an orphaned snippet reference, a section rendering blank, a variant selector not updating price or stock, a layout breaking at one breakpoint.
Several independent issues are not one small fault. They belong in the multiple-issues scope, and I would rather say that up front than let someone believe three separate problems are being priced as one.
It also stops being one small fault when the cause sits outside the theme, which is exactly the case you raised. A conflict across apps, markets, subscriptions or accelerated checkout is not a theme regression and should not be sold as one.
A full pass is every template rendered and read, every reachable page checked for errors, cart and checkout walked on desktop and mobile, and a written list of everything found, including the things I am not fixing.
On your point about a larger dependency surfacing after work has already begun: I stop there and write up what I have actually found before going further, and the scope is agreed again from that point rather than quietly expanding.
On handoff, here is what I do in practice, described as practice rather than as a policy I have not formally written. Work happens on a duplicated theme, so the live one keeps selling throughout. You get a preview link and test it yourself before anything is published. You get it in writing: what was wrong, the root cause, and every file I touched with what changed in it. It is checked on desktop and mobile before I hand it back. Your original theme is untouched and stays in your theme library, so reverting is simply publishing it again, which is in your hands and does not depend on me being available.
I have not set formal terms beyond that, and I am not going to announce a warranty or an SLA I have not properly thought through. That is the kind of promise that is easy to write and hard to honour.
On the metrics, you are right, and they are the ones worth keeping: issue category, time to reproduce, time to fix, whether publication was approved, and whether the same merchant came back with a second problem. I will track them from here. I would just rather tell you I have a sample of zero than present one as evidence.