Friend, you're over-complicating something that's really not complicated at all.
"Nickel" is a personnel scheme. It signifies five DBs. In the Nickel scheme, the "star" is the additional DB. Typically he covers the slot receiver (though not always). Again: nickel is the scheme; star is the position. Some folks do say "nickel back"...when they say that, it is completely synonymous with the "star" position.
"Dime" is another personnel scheme. Signifies six DBs. The sixth DB is sometimes called the "money," though "dime back" is also used.
None of this has anything to do with which backs are "first best" or "third best" or anything like that. Backs are chosen and assigned to particular roles because of their particular combination of skill sets. Lots of variables involved, from speed, to quickness, to height, to knowledge of the defense, to knowledge of the opposing team or even specific opposing receivers. There's really no such thing as "best" and "second best" in all that, so much as one fella is better suited to this job, and that one is better suited to that.
Anyway, none of that has anything to do with terminology. The nickel back is the star; the dime back is the money. Simple.