Yemen is an expensive country for tourism, so most of the visiting tourists are not energetic and rarely have a fervent interest. So it is not surprising that the guides get used to this sort of client and take easy options. As anywhere else in the world, guides rely on tips from satisfied clients so they fail to inform tourists when they are improperly dressed and likely to offend local feelings. Awkward questions, of those to which they do not know the answer, are likely to be answered with a statement that they think you would like to hear, not necessarily the correct one. The bare trip can be accomplished by bus.
Some German tours go father afield but still get back to the hotels at night.
This means very long days, up at 0600 hours and arriving at a hotel at 2100 hours or later. The south can be covered by flying from Sanaa to Aden or Seiyun. Then by road between Mukalla and Seiyun, arriving very late at night, and by road between Aden and Mukalla, also arriving very late at night. There are good roads from Aden to Ta'izz, or direct to Sanaa. You can hire four-wheel drive vehicles in Sanaa, but I would not recommend that you travel to outlying areas alone.