So I've been growing from seed what I thought initially was khat. But after a while I started doubting that it was khat altogether.

After looking around online I think I ended up with a rough Lady's Thumb sprout, likely due to the worm castings I used to germinate (contained various seeds unfortunately).

I'm still pretty convinced it's not khat, but I'm wondering if anyone here has any knowledge on this, and if the pictures are enough for a proper ID. This has been bugging me this month.

It certainly looks much more like a persicaria than a khat plant. Persicaria is a common contaminant of non-sterile compost IME. Maybe try pasteurising your compost, and better luck next time!