Does 225.30 allow a person to have a multiwire 15a. circuit to a residential garage and a 50a. Welder outlet? These would be two separate circuits originating at the house. I say no, but there seems to be others that would allow it.

The letter of the code would prohibit it.
For that matter could you have a multistation smoke/heat detector system tied to the garage separate from the general use circuit.

In your case I would want a sub panel if this was still in the permit/design phase.
The cost difference would be minimal since you could probably still feed the whole mess with 6ga on a 60 from the house.