very good!
Although I'm not really keen to write fluff about my own army (to that level of detail), I very much enjoyed reading that story
tbh, before I read your story I was a little doubtful that there could be a good reason for a Mortifactors/Space Wolves crossover, as both chapters are pretty self-centered, but your story does a very good job of explaining the situation, and I really cant find much wrong with it. (and I can be a right fussy bugger sometimes, as
this will demonstrate...)
As for putting yourself in as Chapter Master - go for it

If your name was Neville or Geoffrey (or David (my name), for that matter) or something equally middle-England Id maybe advise against it, but Sebastian is fine. I can imagine a Sebastian in the 41st millenium, no problem.
Oh, and as for the quality of your English? Seriously, don't worry. Nobody here is Shakespeare, and the fact that it isnt even your native language makes your writing astounding. I am in no way good at languages, and I have discovered this is in a very real and personal way since I came to Japan. My Japanese sucks, and although I dont study much, I have studied enough to understand how much effort and time you must have put in to achieve the level of English that you have. So yeah, don't worry on account of your English. The day we can criticize it in Swedish is the day we can criticize it at all.