Jashin should work the way it did in the anime and manga if i remember correctly.
-Attacking someone has a 25% chance to give you their blood.
-Upon attaining said blood you become black with white markings.(ritual mode or whatever)
--At this stage you are NOT immortal, only ready for a ritual.
-Once a circle is drawn its there for awhile till another is drawn.
--No need to draw another circle because you got someone else's blood.
-Once you have a person's blood you have their blood till either they die or you die.
--Or until you attain someone else's or a clone's blood.
--Which means there no rush.
-The way of Jashin is not for all pain to go to the target, the pain is shared equally meaning what ever the jashin feels, the target feels(while they are on their circle).
--Its now a game of who has more health.
-The way immortality should work is when you kill someone you gain a couple seconds of immortality which stacks with the more people you kill.(a cap somewhere around a minute is highly recommended)
----------------Q&A-----------------
Q: What happens when a jashin with immortality's health hits 0 or less?
A: Nothing, they won't die their health just goes back up to 1.
Q: Would immortality work without being on the circle?
A: Yes, as long as you have time in immortality.
Q: What if I die during ritual without immortality?
A: Then you die and you lose who ever's blood you had.
Q: What if someone else attacks me on my circle?
A: Both you and your target take the damage your dealt.
Q: If I die on my circle does the target whos blood I have die too?
A: If their health hits 0 at the same time yours does yes, otherwise no.
Q: How many seconds of immortality do I get per kill?
A: Maybe about 15 seconds.