Hi Worker Bee,
For backordering, can you simply inflate the quantity on hand? E.g. if you have 100 physical goods then tell Shopify you have 200.
Re Shopify Assistant, the bundling functionality will update inventory for one variant when another variant is sold. In this case you could set the "master" variant to be the $1 one and have every other variant reference it (and set their tracking to "do not track").
Thanks, Gavin.