Moreover I'm finding that it's possible to delete product after order draft is created and then order line loses image, that gives another insight in how it works underneath. Having images for order lines without products would require even more rewo...

Right, now noticed in documentation "To create a custom line item, provide the title, price, taxable, and quantity properties." - did not realize that's completely without product. That way it would be not a trivial feature to make, given the way ima...

If my observations are right, that will never work due to how Shopify is designed. Images need to be uploaded separately to product (max 250 images), and from there they can be linked. Line item seems to be just a variant - so either you create varia...

It is possible. Need to attach image to product (ProductImage API), create product variant with image_id (ProductVariant API), and then create draft order (DraftOrder API) based on variant_id (option1 is mandatory) and not product_id. Otherwise if dr...