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1) You can get a limited cohort from the Recruits or Squire feats and upgrade them to Leadership. Taldan Knight also grants a cohort with specific features and an extra benefit. The Cult Leader and Warlord archetypes grant Leadership for free.
2) They can be. Animal companions are fairly limited in the number of things then can do. Melee damage, flanking, maybe a combat maneuver or two, maybe some kind of scouting, fetching, or message-carrying. Cohorts can do almost as much as a PC can. A well-built support cohort like a bard can be a real force multiplier, and if your GM lets cohorts craft items for the PCs it can get a little crazy. It can also sometimes be challenging to manage roleplaying a cohort in addition to a PC, which is disruptive in a different way.
3) Given the above... yes. They provide much more benefit than you should expect from a feat unless you intentionally severely limit them. However, being overpowered doesn't necessarily make them disruptive. For example, for a party may obtain a cohort healer instead of having a party member take on that role. While this is a very powerful benefit it also helps all party members, not just the person with the cohort, and can make it easier to run a game. The Esquire Cavalier and the Instructor Wizard also look more balanced than the Leadership feat because of the restrictions placed on the cohort and because you give up a mount or familiar, which is a bigger cost than a feat.
4) Looks right to me.