(Going by paragraphs here)
1.) At the moment 'Pure' Mana is simply a relative term, what it truly is is as you put it, but I'm using Pure Mana as its place as it is more broad to the subject and more open to the usual concept of something very powerful, being very deadly.
2.) The object you were thinking of that it would be 'in limbo' with would be the Controlling Object, (EI jewelry etc.), is placed in the Device I mentioned you made me think of, would put it into a sort of stasis that it could no longer be controlled until the Jewelry/object is removed from the Device. As for the second part, the Shard I'm going for is one people can obtain at any time, as long as they have the materials needed, which also includes the willingness of several Voidic Masters, to keep it from becoming an snowflake object. Also, there will be an event I will propose to many of the Major Guilds out there in order to get its presence to be known, without bringing the entire server into a compact place with, what will be no doubt, trolling and people trying to be 'top dog' in the situation over others.
3.) The sort of poisoning from the mana isn't like Dark Magics in a few ways. For instance, it cannot be removed by Holy Magic or any other magic. Only waiting and being healthy, the Voidic Masters who helped created the Shard, and the person in Control of the Shard can remove it. It is also a location and use based poisoning, the closer you are to the Shard, and the more Mana you use in the Shard's presence will increase the rate and damage of the poisoning. As for players ignoring the downsides, I WILL be checking in on the Shard's every now and again to see what has happened to it, as I cannot monitor all things that occur with them, I will watch the things that inevitably remove the Shard from the players control and cause it to be unbalanced and cease to exist with its imploding into the Void.
4.) As I'm not sure I've mentioned before, I haven't gotten all of the Uses out yet, this will include its abilities and whatnot, EI, things only it can do. These may or may not include a strange form, very powerful, yet very limited form of pure void magic.
5.) The Shard, if the 'Jewelry' is tossed and lost from a living person's hand/body, it will cause the Shard to immediately begin to destabilize and, once it has overgone its 'Jewelry's' strength to control it, the Physical Plane will force the Shard and the area surrounding it, into the Void. If the 'Jewelry' is as strong as Tahrah/Estare's Staff is, then it would explode, rather than implode, as at that point the Physical Plane cannot control it and instead of being capable of removing it entirely, it sacrifices the area, rather than the entire Plane.
6.) I hadn't completely thought of that, but it did touch my mind. Though, I believe it would turn back to what I said about paragraph 4. Where it may form a new type of very limited but powerful magic in its area of influence.
7.) As for this, all things are not quite yet explained, I will in time, but a short example of what I was thinking when it 'zapped' people, is it turns the Mana within the Being against itself, breaking it and destroying it, while the Shard's 'aura' effect, fills them with 'Pure' Mana. Essentially, this will be the base ability of the Shard, which would simply be a hyper effect of its 'aura' effect that it already does. As for the thought of it being OP, it isn't really. As stated above the Origin section, the Mortal Shard's will nowhere near be as powerful as the First Shard. Instead, for example, it would take about 5 minutes of an infant of a chipmunk, for the example, to be killed if targeted by the Shard. On normal aura effect, it could take 20-30 minutes of IC gameplay. Being targeted by the Shard only increases the effect by 2 times the original pace. For the reason it reacted differently on the First, was its level of power, and the Staff, being in a temporary placement to slow its growth and effect. Only when Estares had the Staff and commanded it to unleash its strength, did it have a near infinite effect. (But as said, Mortal Shards will be /infinitely/ less powerful than the original). However, after all that, it's base ability is pretty much a side effect that simply increases the damage of its aura.
No idea what happened with the highlight here. I'm trying to sort it out. (Couldn't fix it so I made it as easy as I could to read.)