Inosuke doesn't become a Hashira in the manga. The story progresses with him having other significant experiences and achievements instead of achieving that specific position.
It's hard to give an exact number. Different story arcs and battles led to the deaths of multiple hashira. Some died heroically, while others met their fate in unexpected ways. The count changes as the manga progresses.
Definitely not. Genya remains outside the ranks of Hashira throughout the manga. This is because of a combination of factors such as his skills, experiences, and the overall direction of the story.
The classification of cultivation levels differed from novel to novel. One could see the different levels and corresponding titles. For example, some of the titles mentioned in document [3] included First Stage Transcendent Mortal, Second Stage True Master, Third Stage Battle-King, Fourth Stage Innate, Fifth Stage Spiritual Emperor, Sixth Stage Spiritual Lord, Seventh Stage Spiritual Venerable, Eighth Stage Profound Sage, Ninth Stage Tribulation Transcender, Tenth Stage Immortal, Eleventh Stage Heavenly Dao, and Twelfth Stage Ruler of the Netherworld. However, due to the limited information provided in the search results, it was impossible to know the names of all the immortal cultivation levels.