Question: How realistic is Elliot's plan of building a new plane?
Answer: Completely realistic. As explained correctly in the film, the aerodynamic principles involved are valid. Given that the constructed aircraft would have oversized wing surface area and an excess of power available, it should fly. Disruption of the airflow over the top of the wing due to the passengers would be minimal. In the 1930's airshows featured multiple wing walkers atop much smaller and lower powered aircraft.
Answer: Stupidly unrealistic. The plane simply wouldn't fly with people hanging off its wings for a start.
There's a big difference between a single wing walker on a high lift biplane compared to 10 people hanging off the wings.
Search on "multiple wing walkers" and see a 160 hp biplane carrying 5 walkers. So, for the C-119 there is about 2894 square feet of wing area, call it 2000 after cut down. The PW R-4360 produces 3500 hp, but let's use only 30% of that to protect the cobbled airframe. 10 guys on the wings are going to disrupt airflow over about 12 square feet each leaving about 1880 square feet of unobstructed wing being driven by 1000 hp. 30 people on the wings would not stop it from flying.
Question: Why was the camel left with the 2 murdered men?
Answer: It is stated very plainly that the Arabs left the camel behind because it was lame. This was a dramatic device to demonstrate the Arabs' ruthlessness: They murdered two human beings in cold blood, but allowed a lame camel to live. Note that the camel scene appears in the 1965 film, but not at all in the 2004 film.
Answer: A flying version of the design about 1/2 scale was built and flown for the original 1965 film. It appears in several flying scenes in that movie, but tragically crashed during filming, killing stunt pilot Paul Mantz.