Not sure what you mean 'beta'.
Cell coverage: You may get some at the very top (since you have line of sight down to Lone Pine). Unless they've added towers in the last two years (I hope not), I don't recall getting any coverage anywhere along the trail.
You'll want to download all the maps you want to use ahead of time. Gaia GPS works fine offline as long as you download maps ahead of time.
Sometimes I find that the GPS signal is a little more accurate if I keep the cell radio on (Airplane Mode off). But you eat more battery doing that.
You'll want some additional batteries to recharge your iPhone.
Also, don't rely on the GPS. Have a paper map and know how to use it in case you get lost, the phone runs out of justice, or you drop it off the edge of Whitney's summit.
