Cameron is correct. You cannot bring a specialist keeper on as a sub and have them keep. Whatever the captains agree is moot.
You really can. I remember Bob Taylor maybe five years after his retirement keeping for England in a Test match, to be replaced by Bobby Parks who...well...never got a test cap. I can also remember Richard Blakey coming on to replace Bairstow senior in a game at Scarborough when the latter injured his hand.
There are laws. They stand unless there is an agreement to vary them between the captains that is approved by the umpires. It is only an alteration to the number of players that affects the first class status. Though even that seems to be a moveable feast - after all, how often do England release players to or call them up from Championship matches, for example leading to the ridiculous situation wherein Kevin Innes - a bowler of limited willow wielding skill - made his maiden first class century and was dropped before the match even finished!!!