Balanced vs. unbalanced
Yes, in an unbalanced count the sum of all the weights of the cards in a deck is not zero. So you start with an initial offset which is particular to the number of decks in the game, and when you've gotten to the bottom of the shoe the running count is zero. The card or cards which create the unbalance act as a sort of clock throughout the shoe.
The problem with the indexes is that the initial running count is going to be different for shoes with different numbers of decks. Let's use the most important index as an example, 16 vs 10.
In Hi-Opt II, in this situation you stand when the count is above 1 and draw when it's below 1. Let's say I'm playing a 4 deck shoe, that would correspond to two 5's being removed from the deck. On an 8 deck shoe it would correspond to four 5's being removed.
We'll use KO as an example, 4 deck shoe, the initial running count will be -16 (I think), removing two 5's gives you a RC of -14. If you are playing an 8 deck shoe, the IRC is -32, and removing four 5's gives you a count of -28. But both situations are supposed to be played the same way when you have a 16 vs. 10. So you have to do a division or learn different sets of numbers for different shoe sizes. Wonging out will be a nightmare, you not only have to do a division but estimate what point you are at in the shoe and if you've been playing an unbalanced count all the time you're not going to be good at doing that. Just my opinion, but I'd rather do the division and deck estimation up front, get used to it, then I'll be just fine from DD to 8D.
Hi-Opt II can be played without a sidecount but it's BC isn't very strong that way. A lot of players use letters to keep track of the aces.