To start off with, yes, I got into DnD during the pandemic, and played a fair bit over discord. Haven't played a cleric though, it doesn't fit my aesthetic, but I can still have opinions on character building. So lets get cracking.
1. Wasn't the point that she didn't used to be a dex-based cleric until they revamped her stats, but used the recommended ability spread originally, or did I misunderstand?
My point was that she is dex-based right now, which means that if she attacks with the str-based weapon that she gets as starting gear, she does less damage than if you give her a finesse weapon, but because she doesn't have martial proficiency, if you give her a finesse weapon, she does more damage, but hits less often, as she is denied her proficiency bonus. Unless you give her a dagger, which is only d4, and doesn't really fit with a shield aesthetically. I consider that to be tanking her combat potential.
Your option is to give her a ranged weapon, which isn't part of her starting gear, and thus isn't how she is intended to be played.
Her AC would be trash with a lower dex score, and with the recommended stat spread, she'd have an AC of 13, 15 with a shield, as she'd have a dex modifier of 0, which would be trash for a s&b character. Hence why I assume that them lowering her str and raising her dex was to up her AC, from 13 to 15, giving her 17 with a shield, which is decent, but not great. Compare with if you give her La'zael's half-plate, then she's got 17, 19 with a shield, which is in fact great. This could have been done much easier by either giving her heavy armor proficiency and heavy armor starting equipment, giving her AC 16, 18 with shield, or giving her starting armor with higher AC, like they did for La'zael, half-plate would give her 15, 17 with shield, with the standard stat spread.
Or do you think there was another reason why they switched up dex and str on her character?
2. While I disagree that rule exceptions are necessarily bad, they're named NPCs, pre-built, I have no issues with them having special stuff that other NPCs or the PC doesn't get, they're not necessary to fix Shadowheart.
I just built a trickery cleric on DnDBeyond, and with the recommended stat spread for her class, she'd have 14 AC, 16 with a shield, which is not great, but it's decent, and what the class is based around, because she'd start with scale mail. Of course, they've given her a chain shirt instead, I assume because they want her to be able to sneak without disadvantage. So the trash AC was their own fault for tinkering with her starting gear.
But, Shadowheart is high elf half-elf, and thus has access to Elf weapon training as a racial trait... give her that instead of a cantrip, and a shortsword instead of a mace, and you've fixed her AC, caused by the chain shirt starting gear, by making her dex-based, without tanking her combat ability.
Not to mention clerics can get crossbows as starting equipment, if she had that too, it would not telegraph "melee character" to the player.
She is a trickery cleric with an urchin background... it only makes sense for her to be DEX based. They are practically divine rogues. As for using a STR weapon vs a DEX one... she has a 12 STR and a 14 DEX... that's a 5% difference to hit... that's it. 17 is decent but not great? What D&D campaigns are you playing in? For a T1, medium armor cleric, there's not a thing wrong with a 17 AC. Trickery clerics aren't supposed to be front line tanks.
