Existing, old school carpool programs aren't paying their drivers, either. Which means it isn't commercial use. Which means your regular auto insurance covers it. Unlike this.

They get around this by limiting the amount of payment to the drivers' costs (i.e. the IRS standard mileage rate of $0.54/mile.)

From the TOS:

"...we may charge you to reimburse the Carpool Driver for the costs associated with a Carpool Ride provided to you. [...] Reimbursements may include per mile charges at up to the prevailing IRS standard mileage rate, and other expenses like tolls and parking fees." (Emphasis mine)

That's an interesting legal theory, but I wouldn't want to be the test case.

If they wanted/needed to pay a higher amount, could Lyft get around this by paying the driver two fees - a first constrained one to skirt the insurance rules and then a "separate" fee for another component such as advertising or data provision or something else plausible but arbitrary?

A 1992 email from Richard Stallman about libreadline's license is, oddly enough, relevant here:

"The issue first arose when NeXT proposed to distribute a modified GCC in two parts and let the user link them. Jobs asked me whether this was lawful. It seemed to me at the time that it was, following reasoning like what you are using; but since the result was very undesirable for free software, I said I would have to ask the lawyer.

"What the lawyer said surprised me; he said that judges would consider such schemes to be 'subterfuges' and would be very harsh toward them. He said a judge would ask whether it is 'really' one program, rather than how it is labeled.

"So I went back to Jobs and said we believed his plan was not allowed by the GPL."

Judges are not computers, and are generally capable of seeing through your clever plans.

If it walks like a duck, smells like a duck, and quacks like a duck, a court is very likely to call it a duck.

I think it's debatable, legally, whether or not this constitutes commercial use. It's designed only for picking people up along a route you already drive regularly, and the monthly cap of $400 means no one could conceivably make a living off of it, even if they gamed it somehow and had multiple accounts.

The amounts seem purposely low so as to appear like non-commercial activity.

Pretty sure normal insurance covers giving rides so long as you don't profit -- that is, as long as your compensation is no more than the cost of giving the ride, at which point it would become commercial activity, with a different risk profile.