It's pretty hard to actually clog a PEG as long as you flush it well. I've crushed plenty of pills to make a slurry to give by PEG, sometimes they weren't crushed as well as they might have been, but the big chunks soften up and flush right through anyway. I had forgotten about that! And you're right, warm water works much better. I never saw any difference with soda water.
The water concern would be more with things growing in the tube than making you sick from ingestion, the same water would be going in your stomach without the PEG anyway and nothing magical happens to it in your mouth and throat. But you will be drinking a bit more of the trail water than you normally would, so it could push you over a threshold and make you sick where it wouldn't if you consumed fewer organisms. Consider boiling.
I see no reason why you can't hike while you're eating unless you get regurgitation, but I actually don't know WHY you have the PEG. Your feeds are over an hour? Make sure you check residuals before infusing, sometimes your digestive system acts differently when you start hiking. You're throwing a lot of new variables into the mix.
The electrolytes would primarily be a concern because you could get diarrhea. Try it out for 48 hours at home before trying it in the field!
Something else to consider: how does the hip belt on your pack interact with the PEG tube and insertion site? You definitely don't want any irritation or pulling or leaking.
Here's a link for the Nutren 1.5:
http://www.nestle-nutrition.com/prod...4-9C57D7D69FB5
I also wonder if you might need MORE calories because you're hiking, depends on how long you'll be out.
You know, they do make powdered tube feedings, I see "Emsogen" on the internet. Call a couple of medical supply places and talk to a pharmacist to see about actual availability.