-Yes, charms do make a difference in leveling. crit chance is good too, but crit damage is best utilized when you use back-critting-crystals to multiply your crit dmg. In solo play, as a priest especially, you will be hard pressed trying to attack monsters from behind. Usually for solo leveling the ideal would be power. i hardly run out of mana leveling a priest so infused may not be necessary.
-yes, gathering buffs do stack, and you get an achievement + head costume for maxing out each of the 3 skills. If you max out all skills you unlock an inventory slot row.
Yes, they do make a big difference, but don't buy them on the broker. Buy the green Power Charms from Specialty Stores for 1 gold each.
The only really useful gathering buff is Spirit for mana regen. However, you definitely should level your gathering. Gather all the ores you see - if you have just a few points in alchemy, you can make crit scrolls with them. Keep your extra mats in your bank. At level 55, you'll get a quest which rewards you with an extra line of inventory space if you can give the NPCs 25 of every gatherable material in the game. And if you get all your gathering skills to 300, you'll get even more inventory space.
They do like what they said, I have tested this with a BAMS
With charms 3-5 minutes kill time and huge decrease in damage, without 6-8 minutes with a chance of dying.
When in doubt get power, endurance, infused charms. Works for every situation. I wouldn't get keen for crit when soloing u get a higher chance of doubling ur damage when u hit the mobs from the front. With power u get a consistent increase in damage which is usually higher in a solo leveling situation.