How to increase the resolution of AI images to an acceptable level for stock submissions

The acceptable resolution is 4MP, btw. Which is not that high. 

As you are mentioning increasing the dpi: First you increase the ppi value, second this is the most unnecessary parameter for stock photography. There is only one reason, why this parameter exists: to translate the pixel values into inches. So a picture with 2000points may be interpreted as a 100ppi image, meaning that it will be interpreted as a 20 inch length, or a 200ppi image, meaning that it will be interpreted as a 10 inch length. Nothing, and I repeat nothing has been changed in any quality related parameters. So for convinience, leave the ppi value at 200 or 300, whatever your prefered value is, and don't consider it anymore. It's completely irrelevant.

The resolution, however, is probably the most intersting parameter that you can touch up. But increasing the resolution does not create new information, but that information does need to come from the neighboring pixels and gets interpolated. There has been big progress in the course of the last years in this, increasing the resolution of assets intelligently, trying to keep the sharpness, without introducing new artefacts. But there are limits to this method, as you have experienced.

For some uses, those limits are not important, for some other applications the limits and the loss of quality is a major issue. As for this reason, do not increase the size too much, so that the asset, when checked at 100% still holds the necessary sharpness, without introducing artefacts. But this means that your source imge needs to be perfect, which is rarely the case with generative AI. You need to fix rendering errors first, then scale up and fix upscaling errors and those rendering errors that you did not see during your first checking. It's not al all an easy and fast task.  

Image #1, pearls look slightly off to me.  Also, the bodice creases look odd. For comparison, look up Medieval corsets to get a sense of where the boning structure should be. 

Resolution is just one small part of the equation. 

No AI engine is perfect. They all make mistakes, some worse than others. The more complex an AI artwork is, the greater possibilities for errors.  

Every quadrant of output should be closely examined for accuracy, logical rendering of details & angles, and removal of unwanted artifacts.  When artwork looks synthetic, it's often due to subtle inconsistencies that only humans can detect.

In the first image, the girl has no fingernails, and the weaving of the golden sash down the front of her gown is inconsistent. The image overall appears to be quite noisy, which might have been intentional on your part, but the moderators probably just see it as noise.

All text to image AI models that I'm aware of generate results ranging in the vicinity from 19 to 20 in. on the longest side at 72 ppi. I use Gigapixel AI to upscale my images and have had assets accepted that were as large as 30 inches on the longest side at 300 ppi. (I've since dropped to 20 or 24 inches to minimize subsequent editing).

That said, if I look at your top image at 200%, the people featured on the tapestry are poorly rendered, which would more likely be the reason for refusal. On the second image, the pattern on the puffy shoulder sleeves don't match as well as they probably should, and these are the things moderators appear of late to be taking a closer look at when it comes to AI submissions. So I don't think a lack of resolution was necessarily the reason for these assets to be rejected. But yes, Gigapixel AI is what you'll want to consider looking into when it comes to increasing resolution and retaining (and even improving) sharpness.