I'm not sure about the failed astartes story. I don't think it likely that a space marine chapter just lets a potential neophyte walk away with a valuable implant. I was also under the impression that the implantation process was something you either survive or you don't.
Everything else looks fine, but I think this part would be better if you took out the heart implant part and wrote it that Saul did not make it past the astartes screening process.
I just wanted to touch on this a bit before I post my app. You are pretty much right, but I wanted to throw this out there. Astartes who do not pass evaluation or do not take the implants correctly or who do and fail to take on the tenants of selected legion will either die (because of the failed implants) or they will be put down by the overseeing neophyte leader (usually a scout sergeant). I found this in the first couple chapters of The Outcast Dead
when one of the characters is describing the boss man of the main gangs in the setting.
I will be writing my app up here right now and post what I have completed shortly! Looks like a lot of fun!