As @Smurf2 was saying, line art should be output at a higher resolution than 300 ppi. The 300 ppi thing is to ensure good enough quality for halftones (photos made from 150-lpi halftone dots). Line art (black type, body copy, solid-color lines, etc.) isn’t composed of those dots and should be output at a higher resolution to keep the edges sharp. The typical output resolution of an imagesetter or platesetter is upwards of 3–4000 dots per inch, which is great for line art, but not if it’s already been rasterized to 300ppi. A cheap B&W laser printer output is typically at least 600dpi, so you don’t want to rasterize line art to 300ppi (Yes, dpi, lpi, and ppi are all separate terms that have specific meanings, but that’s a tangent you can look up on your own.)
All that considered, you’re dealing with KDP, which is digital, not offset litho. Their output resolution won’t be anywhere near as high as offset. Another thing to consider is that KDP is the equivalent of a lower-quality gang-run printing. Their whole business model is built around fast and cheap, not quality. If you were dealing with a good offset printer, you’d receive a high-quality (hardcopy, if requested) proof that would enable you to check this stuff with a guarantee of quality. With KDP, under the best circumstances, it’s a crapshoot — you never really know for sure how it’s going to turn out. Considering the relatively low quality, I’m not even sure it matters if you send them a 300ppi, flattened PNG. Honestly, most people won’t even notice the quality difference.
I still think you’re likely worrying about nothing regarding those white stitching artifact lines. They show up quite often on PDFs and lower-resolution proofs, but I’ve never seen them carry over to higher-quality offset output. Then again, as I said, this is KDP, where anything can happen. I suspect that exporting to PDF/X-1a is the best you can do, but I can see why you’re still worried. I’ve only worked with KDP for clients twice, and each time I just sent the clients a PDF/X-1a file. It was like aiming at a target, closing my eyes, firing the gun and hoping it more or less hit the target. I never saw the finished printed books, and I didn’t really want to. At least you’re dealing with KDP instead of IngramSpark, which has some really weird requirements regarding maximum ink densities, but that’s another subject.