Are there any abilities that would defeat or deceive this feature? I currently see NO game abilities, spells, or Spell-like Abilities (monsters usually have these) that adversely effect 'Know your Enemy'.
How aware is the target that it's being assessed? How aware the target is of your assessment of it depends on how you describe (or roleplay) assessing your target of choice. If you are just interacting with the target for 1 minute as per normal interactions such as conversation, shopping, dialogue, or any other non combat interaction? It would have no idea of your comparable knowledge.
Can the Battle Master repeatedly use this feature over the course of four minutes? Yes.
Does that mean you gain insight into two more ability scores each minute you spend? No.
"...if you spend at least 1 minute observing or interacting with another creature..."
This key phrase locks you in to only ever learning two abilities of your choice compared to yours, no matter HOW long you spend observing the creature beyond 1 minute. Even if you stopped observing them and started again, that key phrase remains. Any amount of time spent past 1 minute confers no extra knowledge to you. It would otherwise include the word 'Each' somewhere in there. Such as "for EACH minute you spend observing..." or "EACH time you spend 1 minute or more observing..." . This phrasing let's you know that it is an ability that you can start, stop, and then repeat again to gain additional knowledge. This is not the case as of it's current printing.