One thing that pretty much made me scratch my head was how forgiving Twilight was - aside from Applejack's brief one liner, there were no apologies from anyone, not even Celestia.

So, does anyone know of any stories where Twilight is less-than-forgiving, or where things go grim when Twilight appears with the real Cadence?

Faith and Doubt had a good concept at the start, where Twilight snaps and goes batshit, but the execution itself was poor to piss-poor and the following storylines/sequel are just abominable.

 

Apart from Faith and Doubt, which truly fell apart, I only remember two that have Twilight do anything else.

The first starts with Rainbow coming to say sorry. Rainbow knows what it's like when friends don't stand by you (tMMDW!), and hopes Twilight can forgive them. Twilight breaks down, and says the others bullied her into doing that, and she hadn't wanted to take it away from Rainbow. Rainbow bursts into tears and says the others forced her away from being loyal to Twilight. Then it drops into lesbian clop. Badly written lesbian clop.

The second starts the rest of the M6 sitting in SCC, wondering why they haven't spoken to Twilight recently. Twilight comes in, and buys a doughnut, then leaves without saying hello.
Applejack asks what the problem is, and Twilight blows up - "You all just wanted a pretty princess party, none of you believed me!". Then everyone is really sad, so everyone has hatesex to feel better.

It doesn't seem to be a decent PoD.

 

There's Hard Reset which has Twilight getting caught in a time loop and taking several levels in badass. It has some quirky, funny moments and some moments that are pretty gruesome. Of the Canterlot Wedding stories I've read, which isn't many, I consider it to be my favorite.

 

I find that incredibly disappointing - it's one of best potential point of divergence launch pads out there.

The trauma displayed by Twilight is a pretty gripping read.

 

A Three-In-One that I found memorable, if only because each iteration got progressively more awesome and grim.

A Canterlot Cantata

 

Already read those, though thanks for taking the time to find/recommend!

The one where Cadence died of heartbreak really did make an emotional impact; well-written, indeed.