I think the water up the canyon near EC2/3 is reliable in springtime, it has been good the 4 or 5 years that I’ve visited, including late April of the bone dry 2018.  But I have never been there later than early May. There are some deep pools, so it’s unlikely to be gone completely, but perhaps it might not be fresh and sparkly by mid-May in a dry year.  In any event, it’s not a life-threatening risk when you’re a few miles hike from the trailhead. I would definitely be confident enough in the EC water that I’d hike in with only enough to get me out again if I had to turn around and hike straight back out, expecting a very low probability of having to do that.
So far as I know you still have to use poop bags, and for good reason – the only place you could bury would involve destroying cryptobiotic soil crust, and that’s something they are working hard to preserve.  Even if that wasn’t an issue, it’s a pretty small and well used area that’s easily accessible to many inexperienced hikers, it would get nasty pretty fast.
If you are planning to leave food at the EC campsites while dayhiking, I’d recommend taking in a bear canister or mesh bag, or even a cheap plastic bucket.  There’s no concern about actual bears in the Needles, but it’s the only place where I’ve encountered small critters (I’m not sure what) that were smart enough to go out on a branch and chew through Dyneema cord that was holding up my food hang, to drop the food bag to the ground.