Yeah, you can’t actually do anything to the agent, normally you wouldn’t even know he was there; the only way for him to leave is if he does something and fails - then he gets executed.
You can visit Klingon space, like a tourist… do some beard-watching. There’s not really anything you can do there, but…
And yeah, it’s up to 14 for the Enterprise because of Scotty.
No, all the planets in the home zones are members/satellites, which cannot be influenced; and of course they’ve already been explored, so you can’t explore them.
Otherwise the Klingons would just go to Vulcan an persuade all the Spocks to join them and then win all the space
It should be noted that if the ship gets blown up while it isn’t on an explored planet, everyone on it dies. So probably exploring something would be a good idea?
Also, if that happens, it can come back from a starbase, and pick up the crew again without any spies.
Possibly - but with two ships crippled and the other two under enemy agent control, I’d argue the odds of successful exploration with the “infected” ships is slim. So the Enterprise stays put.