To work around a difficulty or obstacle of some sort is to adopt an approach or manner of working which "goes around" the obstacle rather than confronting it directly.
To shoot in sequence is to film the scenes and shots in the order in which they appear in the script. To save time and money, most productions shoot all the scenes which occur at a given location or on a particular set at the same time, regardless of their order in the script; this means that a particular 'setup' of lights and cameras only has to be effected once instead of over and over again. But shooting out of order demands a very peculiar discipline. Actors and directors must have a very clear sense before a scene is shot of how the entire story will be presented in order to grade their artistic effects towards the appropriate builds and climaxes. Buñuel and his actors had not yet mastered this discipline, so he shot the scenes in sequence to ensure that each scene built on the previous one.
Every foot of film means that Buñuel filmed only one "take" of almost every scene. This is very rare in film and video: ordinarily every scene is shot multiple times, with multiple cameras for each shot. Early takes provide the actors, cameramen and technical personnel rehearsals, so that they can discover any technical difficulties and "cover" any mistakes. And even if a scene goes perfectly in every respect on the first take, a second will be shot as a "safety", in case something happens to the filmstock or tape, or there is some error which nobody has noticed. Films ordinarily shoot at least six to ten feet of film for every foot that is used in the final cut; some directors shoot much more. I find it hard to believe that a near 1:1 shooting ratio helped Buñuel overcome his technical ignorance; this account of the production makes it clear that Buñuel did not stick very closely to his shooting script, but essentially rewrote the film in postproduction, and filled in what he could not shoot himself with purchased stock footage.