Re: About Shinigami and hollows
Its a bit tricky. All specifics aside, the individual needs to transform into an actual Hollow, regardless of being Vizard or possessing the spirit of one inside them like Ichigo. From there, they can probably continue evolving into Gillians and eventually a Vasto Lorde.
Arrancar are the closest Hollows can get to becoming Shinigami without their spiritual being getting decimated and reformed by the Hogyoku, which may not even be possible to begin with.
Ichigo doesn't really "lose" them. At several stages one becomes more dominant over the other, but in the final arc he makes peace with his identity as a real Shinigami and his latent powers and heritage so he relies less on their powers and more on his own.
Ichigo never killed Grand Fisher, so he had time to retreat to Hueco Mundo and become an imperfect Arrancar. Orihime's brother did actually die to Ichigo's Zanpakuto, so his soul was purified and sent to Soul Society.
Shinigami can't naturally become Hollows, because their spiritual makeup is different. It's the same thing as humans trying to become another animal. The Hogyoku is the only way to do it, but doing so warps their mind and makes them more beast-like, which would have happened to Shinji and the other Vizards had Urahara not stabilised their condition. The process of getting a mask is too dangerous to justify the end results. Those who have a mask are lucky.
In all cases bar Ichigo, the Hogyoku was used to hollowfy people, and that is a risky process which only Aizen is capable of doing with the highest chance of success. Ichigo also almost died as well when he got his mask.