I'll to reply to those questions
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Michael Altman , somewhere around 200-300 years ago from the dead space story found some gravitational disorders at one of the place on earth. The locals had a myth about the place long before michael altman decided to dig and find out what exactly is causing this mishap. Regardless, the government sponsored altman and his team to find out what is underneath it. There a black marker was discovered . It had some Dna codes written on it that would alter a human being in a monster for only being around it.
Nobody knows how the marker ended up on earth , some kind of alien technology. The government then went on to research this marker with altman leading the project. Scientists started to change into necromorphs , altman knew of this and the government in order to keep it secret and proceed with the research killed Altman. Unitology was then founded with people calling altman as the founder and marker being the truth of this religion. All sayings of altman that were related to the marker of how it changes human beings and genetic codes was taken into context with necromorphs and the people created a religion not altman.
Research progessed with another person as the head. He also found out about necromorphs and in order to contain it he created a pedestal which stopped the markers effect. Researchers were then able to pull the codings from the black marker and replicate it into a red marker which was man made and buried on aegis 7 which was then put off limits to mining by the earth government. However, over the years ECC (corporation i think) went their and started the events before ds1 . Where the red marker was uncovered , the colony and the ship ishimura was affected . Thus the dna alterations and hallucinations started to spread again.
The earth marker is on a pedestal and not removed yet, red marker was removed.
THe needle helped him into making the bad visions of nicole into good ones. IT changed his hallucinations. Thus was able to destroy the marker. He is also affected by the marker but not in a extreme way.
Make us whole is the creator Isaac clarke and the marker. The convergence event where if clarke had failed he would've turned everyone into more powerful necromorphs. The dna on the marker and it's creator would've then be one.
This is just a short version. You should start with deadspace.wikia . It is a must read for any dead space fan.