Can I use 12" x 12" x 3-1/2" solid concrete blocks as a base under the some of the piers to assist in leveling and/or to reduce the quantity or length of 4x4 leveling? Of course I am assuming tamped ground underneath the solid blocks and that they are level. Any problems or disadvantages with that?

You can use the additional blocks, but you're not gaining a lot. Just expense and time.
You need to use something to level the deck. Trying to level all the blocks to each other is extremely difficult and time consuming. This means you need to cut 4x4's anyways. The longer the 4x4, the easier it is to build the deck.

Though there is not problem with the extra blocks, I would recommend not using them.