Inherit: Once you have maxed out any skill, you can pass it on to everyone else. But since this costs a lot of skill points, it only makes sense with skills that aren't available to everyone. Don't inherit the stamina skill for example, everyone gets that one eventually. Also, inheriting only puts the skill out on the market, but every character still has to acquire and upgrade it separately.
Invisibility: Hunk can make himself invisible for a few seconds. That way he can attack enemies without them fighting back, or better yet, sneak up on them from behind and kill them with a stealth attack. Every time this skill is used, it takes a while to recharge before you can use it again, but the more you upgrade it, the longer you can stay invisible, and the less time it takes to recharge. I have it maxed out already, so now I'm only collecting skill points to inherit it.
Almost all PC players, yes. The Steam forums are full of complaints. I don't know about the consoles, but judging from Darbucor's video, probably not.
Each medallion only counts once, no matter how many of your characters have it. However, as far as I know, if the game asks you to collect ten completion medallions to unlock a character, you have to get them with the same character. Might be the same for the achievement, but I'm not sure. When you open the deploy menu, you can see that some medallions are golden, those are the ones you got with the character you're currently controlling. Silver means that they were acquired using someone else.
