Ok so yes, the worry about impaction is real. It can and will happen if proper precautions are not taken. Usually if a chameleon (or any animal really) does not have proper care, they will be at greater risk of impaction. I think if you use ecoearth it will not be as dangerous as using rocks or sand, but I have seen some concern with this as well. I think that people have thought of loose substrate as a taboo because of impaction stories. However, I think the best option for a chams as far as dirt goes would be to go bioactive. It can be a bit more pricey, but can certainly help with the well-being of your chameleon. You could even make your own using the ecoearth if you wanted. I am not a professional owner, but I have read several articles on this and personally use a bio-active substrate. Though, if you are still worried about impaction, you can't go wrong with paper towels.
Also, I'm sorry to say that your leo is probably just lucky. They actually DON'T live in sandy deserts but rather grassy plains and other arid scrublands. This is just a warning for the future. If you absolutely need to keep the sand I'd recommend dish feeding.