Druses
How did Druses end up not being a Roman soldier anymore? and is there any explanation why he saved Judah's family, other than for Mesala?
In the original story, did Druses really save them?
I'll only answer the final question, since I haven't seen the film.
In the novel, Drusus doesn't come up until Judah returns; he's on Messala's side (enjoying an orgy with him) until he loses the race, and blames him for being indebted to Judah.
The 1959 version has Drusus as Messala's sidekick (maybe more...) at his beck and call. He's indifferent to the Hurs' plight (being the first to arrest them) although he is the one who- via Messala's orders- had them released from the jail.