3) i plan to leave my vehicle at Whitney Portal, shuttle to Horseshoe Meadows to camp the night before i leave, and then pick up my car on return to the Portal. is this a viable option parking-wise? how easy is it to get a shuttle to Horseshoe Meadow campground from Whitney Portal? How is cell phone service at the Portal? (i use Verizon)
You may be able to hitchhike between Whitney Portal and Horseshoe Meadows. I know I've picked up a hiker going the other direction to pick up a car. Also, you can leave on car at each trail head.
I use Verizon and can get a reliable 4G signal walking distance from the portal. You walk to that big turn in the road before the portal where it is line-of-sight to Lone Pine and hang out by the guard rail.
BTW, there is no cell signal at Horseshoe Meadows, but on some of the sections on the road up/down you can get a signal by parking off road.
Alternatively, if you have a Garmin InReach device you can text with it from anywhere.
4) should i expect that it will still be buggy at this time of year?
There are mosquitoes at Cottonwood Lakes. I didn't bother with repellent last year.