Can someone tell me, with this new rule, if a lapped car sent to the tail under caution is it now a lap behind the entire field, or has it been "unlapped"?

Not unlapped, you just move over and let the rest of the cars on the lead lap pass you by before the start. Unlapping would have to be like the "lucky dog" rule in NASCAR where you drive past all the lead lap cars and pace car, otherwise your "giving" a lapped car, a lap they did not run.
The lapped Sprint cars that drop to the tail are already behind the other cars still on the lead lap that they let by. If more than one car a lap down, the lapped cars maintain their position relative to the other lapped cars in the back. As an owner , I like the rule and brings the competitive cars closer at each start.

In most races the guy leading or top 3 work their tails off to put a decent car sometimes a lap down. Not liking it if I am up front leading. And the guy just put a lap down has no chance of getting back on the lead lap if his car starts working hoping for a yellow. When will the double file restarts be implemented?

But what happens when the caution comes out, and the leader is between, for example, 14th & 15th. 14th is still on the lead lap. Does 15th then fall in behind him, and thus be scored a lap behind him?

I believe that is exactly how it works. I know the idea is to put front running cars all together to "add excitement", but aren't USAC races usually exciting enough already. With this, lapped cars might as well just pull in and save their motor.

Am I missing something here? Doesn't USAC have a fairly consistent Move Over Flag policy? Shouldn't the lapped cars be honoring that and yielding to the lead lap cars? What's all the fuss about?

They should but many don't.

Yes.

I believe USAC Sprint features may see more cars parked in the infield after being lapped than under previous rules.

Depends on who the lapped car is Bobo. I doubt KTJ will pull into the infield at TSS even tho he's been lapped the last 3 races there, but for a non USAC regular that's running your probably right.