Welcome to the forums! You'll love the island.
Something very important to know: mid-May is
early spring on the island. Expect there to still be snow and even ice in shaded areas and on the north sides of ridges, unless spring is unusually mild or early. Up on the ridge, there will be more exposure to sun (so less snow) but also more exposure to weather. Be prepared for freezing nights and rainy, even snowy days!
1. Muddiness depends a lot on how spring has progressed. Most of the east end of the trail runs very close to the bedrock. You will definitely find a few muddy places, but those will only be where the ridge runs lowest. I've only ever seen "boot sucking" mud along a few trails near the lake, not the Greenstone.
2. No worry about mosquitoes in mid May. See my comment about this being
early spring. You usually won't have mosquito trouble until mid June.
3. I've never had trouble following the Greenstone. It is rarely on totally bare rock -- most often it's a compacted line of dirt or rock running through grass, brush, and (near the west end especially) through a tunnel of trees. There are more cairns than necessary. (Compare the Minong ridge, where people really miss the trail regularly -- this is nothing like that.)
4. HikingProject has an elevation profile:
https://www.hikingproject.com/trail/700 ... idge-trail
Getting onto and off of the ridge involves 400-ish feet of elevation change and can be quite steep. But even when you're following the ridge, expect nearly constant small ups-and-downs. It can really wear on you if you're not used to it.