I guess so, unless we were severely misinformed. We had historically one big AWS account, we are now splitting it into more fine-controlled accounts and migrated under a "root" account following AWS best-practices to have unified billing and we can enable the Business support only on the big (legacy) account where we run 100% prod and other things. I don't think we had the option to enable it on the root account and get all the other accounts covered.

This is not the case for us. Support is automatically enabled for all the accounts we create in our Organization. Not sure if this depends on company size though

Each AWS account can have it's own support plan.. for a while we had some accounts with a mix of Enterprise and business support.. in fact, every child we made for business support for a while even though we had enterprise support across our AWS organizations

I think you can do that only if you have Enterprise at some point, but I might be wrong