Joseph, thanks for the feedback and reassurance.
Is there way to remedy that "dirty" seamline?
Should I push my fabricator to fix it? Are there potential trade offs in fixing it? Or should I try to hire another fabricator who might be more experienced/capable?

Don't like the bottom of the miter in the third pic either. Joe?

Cpartist, yea the two slabs just don't line up precisely. At least on the front profile facing the living room (first pic), the epoxy hides it well, but the seam gap itself seems larger.
On the back profile facing the kitchen (third pic), the gap itself is amazingly thin (what I was expecting) but then the misalignment is more apparent on the bottom... :'(

Anyway, putting my personal standard and expectation aside, since I admit I'm more particular than most, I also want to be reasonable if this is considered acceptable/fine within the industry.

That's why I'm asking for feedback from people here who might have experienced with these types of work and can help me gauge the fabrication workmanship and hopefully provide some advice on how to proceed.

I'm grateful to have this community space. Thanks.

Clement Ng:

How large a span does the top make and what type of support structure is underneath please? I've posted pictures of estone tops with miters that are self-destructing because of lack of support and proper detailing. Gravity is relentless; it will pull those not-too-bad miters apart. Structures must stand alone; the estone is simply veneer.

I can't imagine a cost effective inconspicuous solution to your apparent contamination.

Joseph, it's a standard size peninsula. The slabs are sitting on top of plywoods above my floor cabinets along a halfway (the peninsula). Is that the information you're asking?

Re: the contaminated seams, is removing and reinstalling the slabs a possibility? The backsplash hasn't gone in yet.

Structurally it should be fine.

You can remove and reinstall the tops, but how does that decontaminate the seams?

I don't understand the context of the first and third picture. What looks like the underside of a build-up? Is this a pix of uninstalled counter? Or an attempt at picture framing meant to be on display?

It’s a waterfall counter installation.

Hillside is correct.