How do I get that 'pre-cut slice' look to actually work when serving?

I did almost this exact thing for my cousin's bridal shower last year, a two-layer 8 inch with one wedge done differently, and I decorated the whole cake intact first. Cutting before you frost is way riskier than it sounds. Even a chilled wedge will slump a little once it's standing alone with no neighbors holding it together, and you'll fight to make the seam invisible once the wedge settles back. Frosting the whole cake as one piece means the outside surface is continuous and the design just sits on top of it. The seam only exists after the first cut, and by then nobody's inspecting it. For the border, yes, keep it back from the actual cut line. I left about half an inch of plain buttercream between my cut lines and the piping, and I'd do the same for your flower border. Half an inch reads as intentional from a distance, and it gives the knife somewhere to go without hitting a petal. If your piped flowers are chunky, maybe a bit more. You can also mark the cut line with a toothpick or a faint scribe line before piping so you know exactly where to keep things clear.

Your overnight chill is genuinely the biggest help here. A cake that's been crumb coated and chilled firm cuts much cleaner, so the special slice comes out with neater edges than you'd expect. I also warmed the knife under hot water and wiped it dry between each cut, which kept buttercream from dragging across the exposed interior.

One thing I'd add: make the special slice the first one you cut, in front of everyone, right when the cake's presented. It keeps mom's wedge pristine for its moment, and any tiny roughness on that cut gets hidden the second it's on her plate. The rest of the cake gets sliced after, and by then the border being slightly imperfect at the edges just doesn't register. So decorate as one cake, mind the half inch gap, lean on the chill, and cut her slice first. It'll read exactly how you're picturing it.

I did a split design for my mom's retirement do and the thing that saved the decorated edge was piping the flowers on little squares of parchment and freezing them, then pressing them onto the cake after the special slice was already out. Then it didn't matter how the knife behaved, nothing got dragged. I used a small offset spatula to place each one and it was way easier than I expected.

One thing to watch is the wedge itself. When you cut mom's piece, run the spatula under it low and slow, chilled cakes can crack at the bottom crust if you rush the lift.

Another trick I've used for cakes like this is piping a line of dots or pearls right on the actual cut line, but making them sit super flat so the knife slides between them. They're basicly sacrificial and they actually help hide the cut edge once the slice is out, so the gap between sections doesn't look bare either. Also, don't forget the wedge's cut faces will show cake layers, not frosting, so if you want her slice pretty from every angle, give those two flat sides a swipe of the pink before anyone sees them.

I'd push back on the hot water knife tip from the first reply. With two colors meeting right at your cut line, a warm blade softens the buttercream and I've watched pink frosting smear straight into grey that way. I did a two-tone christening cake last spring and a cold thin blade, wiped between cuts, kept the edges sharp. The chill does most of the work anyway, so heating the knife fights against it. Also on the wedge-sitting-alone worry, I've cut a section out first and frosted it flat on a board, and it held fine after a half hour back in the fridge. Firm crumb coat, no slumping.