Does anyone have experience with a payment provider that allows one merchant account to operate across multiple stores/domains? Currently, Stripe doesn’t allow this. They seem to want a single storefront/domain to be associated with one LLC, one payment account, etc., so I’m looking for an alternative for a client who wants to offer a white-glove storefront-to-distribution solution for their clients.

That will be difficult since payment providers can have strict KYC standards. If you're pulling in money from another storefront, a business could have KYC concerns. I can see why Stripe may have designed their application like that.

Probably PayPal does it

Stripe Connect doesn’t cut it for you?

PayPal is possibly a good call. Stripe Connect would totally work, except this client wants to prove the business model on a shoestring budget with an almost-no-code solution on Shopify first. We're helping them out with their themes first, and we're going to get them onto Solidus in their next funding round.

This is usually accomplished by baking as a service providers such as IUGU in Brazil. There may be equivalents in the US.