As long as you don't close the game (and don't press the reset button on my Poker) the modified spawns should remain, even if you close Poker; I'm modifying the in-memory version of the file the game uses to determine the spawn times and chances, so unless the game loads the file again the changes should remain. I'm not testing multiplayer, though, because I don't have access to a second Switch.
BTW, the Poker doesn't require Internet per see, just regular networking. Which is different from the Switch's local mode (when it has an L atop the wireless symbol); AFAIK nothing apart from another Switch can connect over lan to the Switch when it's in local mode.