What do you mean by Canon 'digital' lenses do not work with full frame cameras, film or digital? Canon EOS lenses DO work with ALL film and digital cameras. The only difference is the focal length of the lenses. This talk about IRON CLAD agreements is nonsense. A reputable camera dealer will not sell you a lens that will not fit your camera!
There is NO problem with 'softness' or 'flair' on Canon or Nikon and probably not with any of the professional camera makers. I never heard this 'talk' in the thirty years I have beeen a photographer.
Your first smart decision was buying a Canon digital camera. The next smart decision is choosing a Canon EOS lens to compliment your camera. There are over a 100 lenses. You can save a few bucks going with Sigma but the perfect match is Canon camera and Canon lenses.