"...building my own almost-serverless solution ... Netlify headless CMS ... GitHub http API ... markdown + front matter ..."
You don't want your relative to be locked into a hosted solution like Shopify yet you want to create a totally custom solution that probably only you will be familiar with?
No. Do everyone a solid and stick with Shopify. Learn how to create a theme for Shopify and you're set. Or learn how to actually develop for Wordpress+Woocommerce if you want to host it yourself. Both options take care of tons of stuff in the background so you don't have too. Do you really want to spend the next months implementing the most basic ecom functionality?