1) Not sure if you can find a DI without an RO, BUT, if you didn't use an RO system before the DI stage, then I AM pretty sure that the DI would clog and wear out pretty fast. My understanding is this, the RO does the brunt of the work, the DI polishes up after it. I think Air Water Ice is the brand reccomended here, I am purchasing a DI unit for my system when I move next year (along with new stages for practically everything else).
2) I believe you can hook up to the shower head, unscrew the head and screw on the filter. however, you might have to buy some adapters. My RO systems feeder line connection is a pass through, you screw it into the outlet under a sink, and then screw the cold water line into it, completeing the circuit, you might need to find something to close it off.
Hope that helps
"Maybe in order to understand mankind, we have to look at the word itself: "Mankind". Basically, it's made up of two separate words - "mank" and "ind". What do these words mean ? It's a mystery, and that's why so is mankind." ~ Jack Handey