The chord in measure four of the Period worksheet example is identifed as a V(6/4) but it looks like a I(6/4) to me – is that correct or am I missing something?
Also, does the same principle (you introduced in the opening lessons) of starting with the harmony apply here?
What you are referring to is a special case called the Cadential 6/4. While the notes reflect a I6/4, the function is actually an embellishment of the V chord. This is because we are looking at those notes specifically as appoggiaturas or suspensions that resolve down to the their respective 5/4 positions.
For instance, a 6th over the note G is an E, and a 4th is C, but the E resolves down to D (5th above) and the C resolves down to B (3rd above).
And yes, the same principle applies with starting from harmony first. I find it is much easier to start from harmony, especially when beginning composing, however it is useful for a composer at any stage.