if it's so bad, and it's in the front, I'd think they'd be mowing it if it was theirs. Of course, it's on a steep incline for THEM as well, so they probably don't want to do it very often, but it surprises me a lot to think that they'd let it get that bad.
I wonder if they think (or know) that it's yours and are wishing that *you* would cut it.
So you need some facts--like, is that really their yard? That means you need to find the surveyor's marks or get a survey yourself.
If it is, then you say, "We wanted to ask you to please mow that section of your yard that's on the incline. We'd do it, but with the riding mower we're afraid we'd tip over. I think it would be less dangerous with your push mower. And it looks bad for both of us."
If you don't want to pay for the surveyor, you say, "Hey, is this strip yours? We always thought it was, but you never mow it, either. Whose is it?" Then you proceed to the first.
If it's not their lawn, then you'll have to mow it.
In the second situation, no, you don't owe it to them to tell them that you're siccing the police on them. If they have half a brain at all, they know they're rude at the least and endangered as well.
If they were people you knew, I would say that you sort of owe it to them (plus it might be more effective) to say, "I've noticed you and your family and friends don't move out of the road promptly, which is really frustrating for me as a driver, and I would imagine other people as well. And I'm scared for your kids, to be honest." And THEN call. But in this case, feel free to call.