>I need to get a modern map. Will it have a buffer on it to know how far one must go into I Basin on the first night?
Nat Geo does have shaded areas that show the approximate Yosemite wilderness boundaries you have to get beyond. Also, this map shows where you have to reach for each trailhead:
https://www.nps.gov/yose/planyourvisit/upload/wildernesstrailheads.pdfFor your permit you have to get down the trail directly to the west of Mt Starr King past the orange arrow. Once you camp the first night past that arrow, you can mostly go anywhere in the wilderness and camp beyond any of the arrows.You can't exit into Yosemite Valley and enter another trailhead).
>My permit starts May 9. Pretty sure I read that current snowpack is at 25% so I'm wondering what experience anybody can offer for what snow I might have to deal with.
This year, snow shouldn't be a problem in early May around Yosemite Valley (baring us getting some much needed spring snow storms (which can happen - there was a big snowstorm for Memorial Day 2019).
Maybe the higher elevations will still have snow in places, but I suspect that the trails up from the Valley, down to Merced Lake, and down the trails running around the Illilouette Creek will be pretty clear (maybe muddy in places) in early May this year.
There is a chance the Half Dome cables maybe be up in early May this year (need an additional permit). Top of Clouds Rest and anything near Tioga road may still be pretty snowy, but hard to tell until the end of April. Sunrise lakes may be okay. A normal year, I'd say NO to all of these being snow free or even being reachable without snow travel skills, but this has been a horribly bad snow year.
>I have read that June is buggy. Any input on what early to mid May is like?
Yes, mosquitoes will be a problem in May. Get a head net and some good mosquitoes spray.
>I planned to go to Merced Lake but maybe that's not a good idea if I will be mosquito bait constantly.
I'm not sure you can avoid them this May regardless of where you hike.
>I will have to verify with rangers but I'm also wondering if I can go down from Echo Valley to Sunrise Creek, hitting the JMT with my current reservation or will I need a specific JMT auth on the permit when I pick it up (if available).
You need a Happy Isles Pass Through permit with a Donahue Exit permit to hike the JMT from the Valley. Early May? There will be a lot of snow still in the higher elevations (even this year). If you're looking at JMT trail guides, consider early May this year like early June conditions in normal years and plan accordingly. Most people wait till July for the southbound JMT. June start date may very well be possible for most people this year.
Even this year, Tioga Road probably won't be open in early May, so starting from Lyell Canyon with a Donahue Exit permit probably wont' be doable (but stranger things have happened).