Of course I am. I dare not meddle with things that aren't my business. You ought do the same, some time.
Many of these questions are answered within the thread. You simply have to read it carefully. Any being may use Grim, thought it would be of no use to them, as the only written purpose is to construct these puppets. The puppeteer uses his own soul to do so, his "grim". In regards to RP, not much will change besides the creation of the puppet. This is only existent to provide a more complicated way to go about creating the vessels used to curse a victim.
On to your questions:
Grim comes from you, the puppeteer. It is the solidified connection from your soul to the doll.
Any being has access to it, inherently, though SP is the only magic that uses it, rendering it useless otherwise.
It is the puppeteers soul. Imagine it as a monorail for the curses to travel through. Curse follows the grim. Hits the doll. Doll is connected to it's victim via the victims blood, as done in the lore. Grim is ONLY to create YOUR connection to it. No random joe may be cursed, as their blood is necessary to create the link between them and the doll. You: Need Grim. They: Need blood.
The needle, when sewn through the doll, leaves the grim behind, as if it were leaving a trail.
The purpose of locking it is in hopes of limiting the use of such a powerful magic, given the servers habit of metagaming.
All roleplay prior may still have occured. Until accepted, all RP will still occur as such. When, if accepted, this process will presumably be the "way to do it".