But to answer the first, I’ve heard directly from party strategists that they look for people who vote, but not in a particular race. They can’t identify them directly, but a higher ballot submitted count than (eg) presidential vote count is a signal that they can gain voters in that area — which they follow up by surveying independents, etc to see what policy issues they’re concerned with.

The argument is that by not voting some rounds, you influence their platform in subsequent rounds. If you vote for them regardless, there’s no incentive to optimize their platform to address your concerns.