The steelcase Leap with a 5" cylinder goes down to 15.5" seat pan height on their standard models. Do you have any wiggle room on the 15" measurement?

https://www.amazon.com/dp/B00GBUPUOY

Alternatively companies like Bodybilt (now Ergogenesis) and Sitmatic have cylinder options that put the seat pan height down to 13.5". Check out the Nano Cylinder on P10 of the Sitmatic catalog

https://sitmatic.com/wp-content/uploads/2019/01/SitmaticPricelist2019.pdf

The challenge with both those companies is they have SO MANY options that it can be really hard to figure out exactly what to get. Their standard chairs can go down to 13.5" though. Your best bet is to call them and talk to a rep. That way you can tell them you need to start with a seat pan height that'll go down to 15" and they can then talk you through the rest. Of course, they're also pricey but it sounds like you really need a unique solution.

Alternatively, most companies (except Steelcase) use the same basic type of cylinder to adjust seat height. If you already have a chair that you like but doesn't go low enough, chances are you can find a replacement cylinder on the internet that goes shorter and do the replacement yourself. I just did a quickie search on amazon and found this one

https://www.amazon.com/MySit-Cylinder-Replacement-Warranty-Universal/dp/B07DLS2KG6

that goes really low and even has the quickie tutorial in the pictures on how to change your own cylinder. I've done cylinder swaps (repairing chairs when the cylinder dies) and they're not too bad if you're mechanically minded (or know someone who is).

As for reducing seat height by removing the casters, most do just pull off so you can do that. Companies sell glides that fit into the same socket and aren't as tall as the wheels. A glide is typically 1" and the chair wheels are typically 2.8, so the net 1.8" shrink should give you the height you need. However, YMMV with small variations between the wheels on those amazon chairs vs the chairs I'm used to (like the above high-end models). Also, you'll want to check the diameter of the stem on the castor once you pull one off. There are typically 2 major types of stems.