any of them beyond the normal Bandits should make it more difficult, although the amount of difficulty I don't know about. Simply, by making the Bandit an Infamous Gang, or using the Undead Outlaws (which are slightly more difficult than the regular outlaws), or using an Infamous Undead Outlaw or Scafford Gang, should make it a little more difficult. I don't think any of them will increase the difficulty too much, though.
This is true. But might also be ok. The city missions scale terribly compared to the rest of the game.
Including harder outlaws (in the form of scafford or undead) might be exactly what the frontier doc ordered.