How does the Fleeca heist affect the Loyalty challenge

The point of the feature is to enable one to one voice calls from inside the game(I have succesfully used this feature to talk with friends). It doesn't matter if you're in SP or MP. The reason why it costs 10$ is to add immersion and to act as a yet another money sink in GTA online.

purpose of stealth is to sneak up on enemies and use the one hit kill attack.. stealth skill is gained by staying in stealth mode in-game (walking in stealth) and it seems to serve little purpose compared to single player because as you said, GTAonline seems to group a bunch of enemies together rendering stealth almost impossible, as one shoot one and the rest become terminators ready to kill with vision scopes of thousands of miles away knowing your exact location. Therefore your style of stealth, while being theoretically very smart, doesn't work in this game. You might have a better chance sneaking up on human players or using it during death match

source : level 87 in gta online and the experience that comes with time.