Lying is a strong word isn't it? I never said Jim was lying ...
Not at all... not in this context.
Every story Jim tells is, at some level, a "lie" -- an un-truth, spoken/written in full knowledge that it isn't true.
I used to tell my kids, when they were little:
I try really, really hard never to lie to you. I don't always tell you every detail, and everybody makes errors and is mistaken about some stuff... but I don't lie to you.
Except once a year, at Christmas.
You're getting coal for Christmas.
Jim lies to us to entertain us. And sometimes -- iirc by his own admission -- he even lies outside of "canonical" stories, in the Q&A / AMA / etc that we collect as "WoJ"'s, when he seems it sufficiently-important to keep a plot-element back as a surprise and just saying "I won't tell you" is insufficient.
But back to the point at hand: Mort's power had indeed weakened, confirmed by Jim:
... his talent started failing him from time to time–which affected his confidence, which in turn made his talent even shakier, a vicious cycle ...
Or:
... what I read or interpret what I am reading in the text ... because it is fiction it is open to many interpretations from the readers ... who knows when he answered that question he may have been after one thing, but later on in the series he might change his mind, we've seen examples of that as well ...
Because Jim engages with us outside the context of his art, we have more opportunity to query -- even have dialogue with -- the artist & his intent. We're always free, of course, to take the work entirely on the basis of its own merits & our own responses, and disregard the artist themselves!
Personally, I prefer to consult the WoJ's to further my understanding.
But, of course, you're right -- Jim changes his mind. And the series is just as subject to "early episode weirdness" as any other. And sometimes (albeit rarely) Jim even lies directly.
Even so, I like to have the WoJ's, and I read them when I find new ones!
... Train somewhat? Badly trained? I'd like to repeat a statement my brother in law used to love to make, in other words, "Charity knew just enough to be stupid!" The same could go for Kim if you will remember?
You had alleged Charity was "untrained," I just thought it an important distinction that -- however piss-poor he was -- there
was a magic-trainer in Charity's life. An abusive one, without much native power; but she wasn't entirely untrained.
... I think talent is genetic, she managed to pass it on to at least one of her children, didn't she? ...
The WC calls it "Salic Law" -- mothers with magical gifts tend to have children with magical gifts; apparenty it's not a male-line (Y-chromosome?) thing, however.
But sometimes power "comes from nowhere," no known talent from ancestry; and sometimes the most promising of parentage produces no more than a squib. Margaret LaFey was the daughter of a mundane mother. I think "recessive" genes and "masking" genes are part of the issue; but there's more...
Jim has made it clear that magical talent is (at least in part) a matter of
epigenetics:
https://harvardcenter.wpenginepowered.com/wp-content/uploads/2019/02/EpigeneticsInfographic_FINAL.jpg.
Molly is talented because Charity's potential got expressed; and that happened specifically because Charity had relatively-recently been a practitioner herself (and (maybe) because she'd been exposed to other magic (her coven/cult, the dragon Siriothrax)). The rest of the Carpenter kids did
not have their potential expressed... specifically because Charity had been working so diligently to
suppress her magic and had no other magic influencing her.
(this kind of environmental exposure causing different "expressions" of similar/identical genetics is what the study of epigenetics is all about)
As adults, Molly has full power, while her younger sibs would need something extraordinary to even get minor abilities; their potential is akin to most mortals'.
... Charity's kids couldn't stir up any talent, or at least not enough to make that much difference, if they didn't have the talent to begin with...
Jim explicitly says the other kids aren't very different from most humans in the Dresdenverse:
... that’s most of humanity in the Dresden Files, really. Everyone has some kind of ability, if they just want to look hard enough to find it ...
... Which kind of contradicts what he said about Mort doesn't it? Or maybe having it both ways to give himself as an author more flexibility, I understand that.
Charity's situation and Mort's are entirely different.
Mort never abandoned his powers; he just lost faith in himself, and they weakened... but never faded away to zero. When Mort started believing in himself and his powers again, he was able to rebuild them.
Charity actively abandoned her powers; she chose to let them atrophy, and fade... and now they have actively faded beyond her reach (as witness her later children being born without magic talent).