Thoughts:
As far as hiking apps, I've tried a few, but don't bother anymore since even if the app worked, my phone battery would probably die before I finished the hike. Better plan: familiarize yourself with a $10 map and compass. Also, cellular based route finding is not the same as satellite based GPS; i.e. don't count on consistent cellular coverage in any mountains.
If you're planning on the Main Trail, there is a fairly well developed trail essentially to the summit which will be hard to miss even in the dark under headlamp. You'll also have plenty of other "companions" in August on the trail.
For your acclimatizing plan, rather than downhill mountain biking, try uphill walking/hiking to 11,12,13,14k ft to better prepare yourself for Whitney. If you're bent on staying on the bike, then at least ride up the hills to better build your aerobic capacity. I'd also suggest to NOT do four days of stressful aerobic work during the four days preceding your planned climb and rather be doing that work now until mid-August, leaving the four days before your climb to acclimatize to the altitude and do some easier walks.
Given your comment on your planned hiking group, reconsider your hiking companions or educate them beforehand and take them on some easier hikes to warm up in the weeks before your trip to assess their fitness level and how they are at altitude and how they manage being on a trail all day at higher elevations. You don't want to find yourself in a position to have to manage a couple of people getting AMS on the backside of Trail Crest.
Enjoy your trip!
