Here's my take: Mueller can investigate issues that "arise from" his investigation of Russian interference. One of those issues is "why" members of the Trump campaign and others might have coordinated with Russians or others to compromise the election. Thus Paul Manafort's and Rick Gates' financial entanglements and crimes were within his ambit. If the same or similar issues arise with regard to Trump, the Trump organization, or Trump family members, it would appear to be within his authority, as endorsed by several federal courts.