If you have good karma, it seems the only way to successfully do the quest is to make no mention of the offer from Caesar or ask about jobs to do as both immediately set the quest stage for Render as "Couldn't convince the White Glove Society" or something like that. However, if you only go for the speech check without first doing anything else (which gives Mortimer no indication that Caesar is making an offer) does it seem to work correctly. This seems very strange to me. I would think that if Mortimer knew you were working for Caesar, he would bypass the karma/speech check, or that the option for the offer of alliance would be available after you had successfully completed Beyond the Beef. Although, it could be just me that has encountered this problem.