This is the only mode I played on the other COD games.  I hate playing normal mode and am just grinding through it to get to 19 so I can play on hardcore.  If you prestige, do all of your game modes lock again or will they all stay open since you've already unlocked them?

They stay open.

That seems like a ridiculous level to be locked at. Why lock it to begin with?

they stay open once you prestige.

They probably made it like this so that low leveled players don't kill their teammates constantly by accident because maybe Treyarch thinks that new players have no idea who's on their team and will shoot anyone on sight which will then piss off many players. Or maybe because some people make extra accounts just to mess with people on hardcore for teh lulz. This would prevent these annoyances. Though I'd have to agree.. 19 is a bit high.