Here's what I've decided to do. After reading some articles online about these topics, I'm not sure if it is ideal, although it doesn't seem incorrect.
I have two narrators in the book. I've decided to use a slightly different approach for each.
The primary narrator is Detective Rob Stack. I'm using First-person present tense for Rob.
The secondary narrator is the omniscient viewer. She narrates all of the scenes that Rob is not around to see. I've decided that the omniscient viewer should narrate in the Third-person, past tense.
This way, it will be easy for the reader to tell who is narrating a scene - Rob, or not-Rob.
I've thought about changing the book so that Rob can be in all of the scenes - but this is far too limiting for the plot's development. I've thought about just using the omniscient narrator - but that has character development limitations I don't like as well.
Anyway, I just spent the last few hours going through the entire book to be sure that the tense is correct for each scene, using my strategy above.
(When I'm writing, I'm not very good at sticking to the correct tense. I'm sure I still have tense issues that will need to be caught in edit.)
Anyway, if you'd like to download the version with tense corrected - it is online now!